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Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 562,695      
Initial Cost [Abstract]        
Land 589,391      
Buildings and Improvements 3,246,749      
Net Improvements (Retirements) Since Acquisition 651,448      
Carrying Amount at Which Carried [Abstract]        
Land 619,181      
Buildings and Improvements 3,868,407      
Total 4,487,588 $ 4,631,128 $ 4,669,289 $ 4,726,169
Accumulated Depreciation at December 31, 2015 1,080,616 $ 1,078,996 $ 983,808 $ 954,665
PENNSYLVANIA SUBURBS [Member] | 400 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances 0      
Initial Cost [Abstract]        
Land 2,657      
Buildings and Improvements 4,462      
Net Improvements (Retirements) Since Acquisition 13,383      
Carrying Amount at Which Carried [Abstract]        
Land 2,657      
Buildings and Improvements 17,845      
Total 20,502      
Accumulated Depreciation at December 31, 2015 $ 6,357      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 1999      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 300 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,206      
Buildings and Improvements 13,422      
Net Improvements (Retirements) Since Acquisition 3,896      
Carrying Amount at Which Carried [Abstract]        
Land 2,206      
Buildings and Improvements 17,318      
Total 19,524      
Accumulated Depreciation at December 31, 2015 $ 8,603      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1050 Westlakes Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,611      
Buildings and Improvements 10,445      
Net Improvements (Retirements) Since Acquisition 5,056      
Carrying Amount at Which Carried [Abstract]        
Land 2,611      
Buildings and Improvements 15,501      
Total 18,112      
Accumulated Depreciation at December 31, 2015 $ 8,686      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1999      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1200 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,595      
Buildings and Improvements 11,809      
Net Improvements (Retirements) Since Acquisition 3,533      
Carrying Amount at Which Carried [Abstract]        
Land 2,595      
Buildings and Improvements 15,342      
Total 17,937      
Accumulated Depreciation at December 31, 2015 $ 7,454      
Real Estate And Accumulated Depreciation Construction Year 1994      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 200 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,533      
Buildings and Improvements 9,460      
Net Improvements (Retirements) Since Acquisition 2,178      
Carrying Amount at Which Carried [Abstract]        
Land 1,533      
Buildings and Improvements 11,638      
Total 13,171      
Accumulated Depreciation at December 31, 2015 $ 5,846      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1180 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,086      
Buildings and Improvements 8,342      
Net Improvements (Retirements) Since Acquisition 2,978      
Carrying Amount at Which Carried [Abstract]        
Land 2,086      
Buildings and Improvements 11,320      
Total 13,406      
Accumulated Depreciation at December 31, 2015 $ 3,952      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 100 Berwyn Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,180      
Buildings and Improvements 7,290      
Net Improvements (Retirements) Since Acquisition 1,948      
Carrying Amount at Which Carried [Abstract]        
Land 1,180      
Buildings and Improvements 9,238      
Total 10,418      
Accumulated Depreciation at December 31, 2015 $ 4,696      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1160 Swedesford Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,781      
Buildings and Improvements 7,124      
Net Improvements (Retirements) Since Acquisition 6,108      
Carrying Amount at Which Carried [Abstract]        
Land 2,045      
Buildings and Improvements 12,968      
Total 15,013      
Accumulated Depreciation at December 31, 2015 $ 3,689      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1100 Cassett Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,695      
Buildings and Improvements 6,779      
Net Improvements (Retirements) Since Acquisition 1,272      
Carrying Amount at Which Carried [Abstract]        
Land 1,695      
Buildings and Improvements 8,051      
Total 9,746      
Accumulated Depreciation at December 31, 2015 $ 2,847      
Real Estate And Accumulated Depreciation Construction Year 1997      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 980 Harvest Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,079      
Buildings and Improvements 8,315      
Net Improvements (Retirements) Since Acquisition 149      
Carrying Amount at Which Carried [Abstract]        
Land 1,949      
Buildings and Improvements 8,594      
Total 10,543      
Accumulated Depreciation at December 31, 2015 $ 3,069      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 910 Harvest Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,225      
Buildings and Improvements 8,645      
Net Improvements (Retirements) Since Acquisition (718)      
Carrying Amount at Which Carried [Abstract]        
Land 1,225      
Buildings and Improvements 7,927      
Total 9,152      
Accumulated Depreciation at December 31, 2015 $ 4,822      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 925 Harvest Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,671      
Buildings and Improvements 6,606      
Net Improvements (Retirements) Since Acquisition 913      
Carrying Amount at Which Carried [Abstract]        
Land 1,671      
Buildings and Improvements 7,519      
Total 9,190      
Accumulated Depreciation at December 31, 2015 $ 3,539      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 920 Harvest Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,208      
Buildings and Improvements 6,595      
Net Improvements (Retirements) Since Acquisition (427)      
Carrying Amount at Which Carried [Abstract]        
Land 1,208      
Buildings and Improvements 6,168      
Total 7,376      
Accumulated Depreciation at December 31, 2015 $ 2,892      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 181 Washington Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,927      
Buildings and Improvements 14,722      
Net Improvements (Retirements) Since Acquisition 318      
Carrying Amount at Which Carried [Abstract]        
Land 6,237      
Buildings and Improvements 15,730      
Total 21,967      
Accumulated Depreciation at December 31, 2015 $ 1,215      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 426 Lancaster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,689      
Buildings and Improvements 6,756      
Net Improvements (Retirements) Since Acquisition 376      
Carrying Amount at Which Carried [Abstract]        
Land 1,689      
Buildings and Improvements 7,132      
Total 8,821      
Accumulated Depreciation at December 31, 2015 $ 3,621      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 52 Swedesford Square [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,241      
Buildings and Improvements 16,579      
Net Improvements (Retirements) Since Acquisition 1,852      
Carrying Amount at Which Carried [Abstract]        
Land 4,241      
Buildings and Improvements 18,431      
Total 22,672      
Accumulated Depreciation at December 31, 2015 $ 8,115      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | One Progress Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,399      
Buildings and Improvements 5,629      
Net Improvements (Retirements) Since Acquisition 3,802      
Carrying Amount at Which Carried [Abstract]        
Land 1,127      
Buildings and Improvements 9,703      
Total 10,830      
Accumulated Depreciation at December 31, 2015 $ 4,883      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 500 Enterprise Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,303      
Buildings and Improvements 5,188      
Net Improvements (Retirements) Since Acquisition 3,709      
Carrying Amount at Which Carried [Abstract]        
Land 1,303      
Buildings and Improvements 8,897      
Total 10,200      
Accumulated Depreciation at December 31, 2015 $ 4,624      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 640 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,222      
Buildings and Improvements 16,891      
Net Improvements (Retirements) Since Acquisition 4,760      
Carrying Amount at Which Carried [Abstract]        
Land 4,222      
Buildings and Improvements 21,651      
Total 25,873      
Accumulated Depreciation at December 31, 2015 $ 10,262      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 555 Croton Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,486      
Buildings and Improvements 17,943      
Net Improvements (Retirements) Since Acquisition (228)      
Carrying Amount at Which Carried [Abstract]        
Land 4,024      
Buildings and Improvements 18,177      
Total 22,201      
Accumulated Depreciation at December 31, 2015 $ 7,337      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 630 Allendale Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,836      
Buildings and Improvements 4,028      
Net Improvements (Retirements) Since Acquisition 12,595      
Carrying Amount at Which Carried [Abstract]        
Land 2,898      
Buildings and Improvements 16,561      
Total 19,459      
Accumulated Depreciation at December 31, 2015 $ 6,083      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 620 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,770      
Buildings and Improvements 11,014      
Net Improvements (Retirements) Since Acquisition 3,700      
Carrying Amount at Which Carried [Abstract]        
Land 2,770      
Buildings and Improvements 14,714      
Total 17,484      
Accumulated Depreciation at December 31, 2015 $ 8,116      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1000 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,772      
Buildings and Improvements 10,936      
Net Improvements (Retirements) Since Acquisition 1,912      
Carrying Amount at Which Carried [Abstract]        
Land 2,772      
Buildings and Improvements 12,848      
Total 15,620      
Accumulated Depreciation at December 31, 2015 $ 5,898      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1060 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,712      
Buildings and Improvements 10,953      
Net Improvements (Retirements) Since Acquisition 3,585      
Carrying Amount at Which Carried [Abstract]        
Land 2,712      
Buildings and Improvements 14,538      
Total 17,250      
Accumulated Depreciation at December 31, 2015 $ 6,418      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 630 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,773      
Buildings and Improvements 11,144      
Net Improvements (Retirements) Since Acquisition 1,991      
Carrying Amount at Which Carried [Abstract]        
Land 2,773      
Buildings and Improvements 13,135      
Total 15,908      
Accumulated Depreciation at December 31, 2015 $ 6,403      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1020 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,168      
Buildings and Improvements 8,576      
Net Improvements (Retirements) Since Acquisition 7,627      
Carrying Amount at Which Carried [Abstract]        
Land 2,168      
Buildings and Improvements 16,203      
Total 18,371      
Accumulated Depreciation at December 31, 2015 $ 7,618      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1040 First Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,860      
Buildings and Improvements 11,282      
Net Improvements (Retirements) Since Acquisition 5,037      
Carrying Amount at Which Carried [Abstract]        
Land 2,860      
Buildings and Improvements 16,319      
Total 19,179      
Accumulated Depreciation at December 31, 2015 $ 6,079      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 610 Freedom Business Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,017      
Buildings and Improvements 8,070      
Net Improvements (Retirements) Since Acquisition 2,477      
Carrying Amount at Which Carried [Abstract]        
Land 2,017      
Buildings and Improvements 10,547      
Total 12,564      
Accumulated Depreciation at December 31, 2015 $ 5,363      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 650 Park Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,916      
Buildings and Improvements 4,378      
Net Improvements (Retirements) Since Acquisition 1,655      
Carrying Amount at Which Carried [Abstract]        
Land 1,916      
Buildings and Improvements 6,033      
Total 7,949      
Accumulated Depreciation at December 31, 2015 $ 3,129      
Real Estate And Accumulated Depreciation Construction Year 1968      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 500 North Gulph Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,303      
Buildings and Improvements 5,201      
Net Improvements (Retirements) Since Acquisition 1,445      
Carrying Amount at Which Carried [Abstract]        
Land 1,303      
Buildings and Improvements 6,646      
Total 7,949      
Accumulated Depreciation at December 31, 2015 $ 3,577      
Real Estate And Accumulated Depreciation Construction Year 1979      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 620 Allendale Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,020      
Buildings and Improvements 3,839      
Net Improvements (Retirements) Since Acquisition 628      
Carrying Amount at Which Carried [Abstract]        
Land 1,020      
Buildings and Improvements 4,467      
Total 5,487      
Accumulated Depreciation at December 31, 2015 $ 2,132      
Real Estate And Accumulated Depreciation Construction Year 1961      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 600 Park Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,012      
Buildings and Improvements 4,048      
Net Improvements (Retirements) Since Acquisition 385      
Carrying Amount at Which Carried [Abstract]        
Land 1,012      
Buildings and Improvements 4,433      
Total 5,445      
Accumulated Depreciation at December 31, 2015 $ 2,166      
Real Estate And Accumulated Depreciation Construction Year 1964      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 660 Allendale Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 396      
Buildings and Improvements 948      
Net Improvements (Retirements) Since Acquisition 2,957      
Carrying Amount at Which Carried [Abstract]        
Land 1,085      
Buildings and Improvements 3,216      
Total 4,301      
Accumulated Depreciation at December 31, 2015 $ 1,219      
Real Estate And Accumulated Depreciation Construction Year 2011      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 640 Allendale Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 439      
Buildings and Improvements 432      
Net Improvements (Retirements) Since Acquisition 1,507      
Carrying Amount at Which Carried [Abstract]        
Land 439      
Buildings and Improvements 1,939      
Total 2,378      
Accumulated Depreciation at December 31, 2015 $ 808      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 14 Campus Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,244      
Buildings and Improvements 4,217      
Net Improvements (Retirements) Since Acquisition 1,533      
Carrying Amount at Which Carried [Abstract]        
Land 2,244      
Buildings and Improvements 5,750      
Total 7,994      
Accumulated Depreciation at December 31, 2015 $ 3,665      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 17 Campus Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,108      
Buildings and Improvements 5,155      
Net Improvements (Retirements) Since Acquisition (374)      
Carrying Amount at Which Carried [Abstract]        
Land 1,108      
Buildings and Improvements 4,781      
Total 5,889      
Accumulated Depreciation at December 31, 2015 $ 1,939      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 11 Campus Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,112      
Buildings and Improvements 4,067      
Net Improvements (Retirements) Since Acquisition 997      
Carrying Amount at Which Carried [Abstract]        
Land 1,112      
Buildings and Improvements 5,064      
Total 6,176      
Accumulated Depreciation at December 31, 2015 $ 2,116      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 1999      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 15 Campus Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,164      
Buildings and Improvements 3,896      
Net Improvements (Retirements) Since Acquisition 247      
Carrying Amount at Which Carried [Abstract]        
Land 1,164      
Buildings and Improvements 4,143      
Total 5,307      
Accumulated Depreciation at December 31, 2015 $ 1,519      
Real Estate And Accumulated Depreciation Construction Year 2002      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 18 Campus Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 787      
Buildings and Improvements 3,312      
Net Improvements (Retirements) Since Acquisition 478      
Carrying Amount at Which Carried [Abstract]        
Land 787      
Buildings and Improvements 3,790      
Total 4,577      
Accumulated Depreciation at December 31, 2015 $ 1,649      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 401 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,199      
Buildings and Improvements 16,131      
Net Improvements (Retirements) Since Acquisition 16,709      
Carrying Amount at Which Carried [Abstract]        
Land 6,199      
Buildings and Improvements 32,840      
Total 39,039      
Accumulated Depreciation at December 31, 2015 $ 11,265      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 4000 Chemical Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,373      
Buildings and Improvements 24,546      
Net Improvements (Retirements) Since Acquisition 1,484      
Carrying Amount at Which Carried [Abstract]        
Land 4,373      
Buildings and Improvements 26,030      
Total 30,403      
Accumulated Depreciation at December 31, 2015 $ 5,570      
Real Estate And Accumulated Depreciation Construction Year 2006      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 610 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,651      
Buildings and Improvements 14,514      
Net Improvements (Retirements) Since Acquisition 3,375      
Carrying Amount at Which Carried [Abstract]        
Land 3,651      
Buildings and Improvements 17,889      
Total 21,540      
Accumulated Depreciation at December 31, 2015 $ 6,508      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 600 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,652      
Buildings and Improvements 15,288      
Net Improvements (Retirements) Since Acquisition 1,985      
Carrying Amount at Which Carried [Abstract]        
Land 3,652      
Buildings and Improvements 17,273      
Total 20,925      
Accumulated Depreciation at December 31, 2015 $ 5,902      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 630 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,558      
Buildings and Improvements 14,743      
Net Improvements (Retirements) Since Acquisition 997      
Carrying Amount at Which Carried [Abstract]        
Land 3,558      
Buildings and Improvements 15,740      
Total 19,298      
Accumulated Depreciation at December 31, 2015 $ 5,537      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 620 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,572      
Buildings and Improvements 14,435      
Net Improvements (Retirements) Since Acquisition 963      
Carrying Amount at Which Carried [Abstract]        
Land 3,572      
Buildings and Improvements 15,398      
Total 18,970      
Accumulated Depreciation at December 31, 2015 $ 5,551      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 660 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,694      
Buildings and Improvements 5,487      
Net Improvements (Retirements) Since Acquisition 19,418      
Carrying Amount at Which Carried [Abstract]        
Land 5,405      
Buildings and Improvements 23,194      
Total 28,599      
Accumulated Depreciation at December 31, 2015 $ 2,357      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2012      
Depreciable Life (Years) 30 years      
PENNSYLVANIA SUBURBS [Member] | 2240/2250 Butler Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,104      
Buildings and Improvements 4,627      
Net Improvements (Retirements) Since Acquisition 1,312      
Carrying Amount at Which Carried [Abstract]        
Land 1,104      
Buildings and Improvements 5,939      
Total 7,043      
Accumulated Depreciation at December 31, 2015 $ 3,888      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 2260 Butler Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 662      
Buildings and Improvements 2,727      
Net Improvements (Retirements) Since Acquisition 2,009      
Carrying Amount at Which Carried [Abstract]        
Land 550      
Buildings and Improvements 4,848      
Total 5,398      
Accumulated Depreciation at December 31, 2015 $ 2,460      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 120 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 685      
Buildings and Improvements 2,773      
Net Improvements (Retirements) Since Acquisition 667      
Carrying Amount at Which Carried [Abstract]        
Land 685      
Buildings and Improvements 3,440      
Total 4,125      
Accumulated Depreciation at December 31, 2015 $ 1,861      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 140 West Germantown Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 482      
Buildings and Improvements 1,976      
Net Improvements (Retirements) Since Acquisition 282      
Carrying Amount at Which Carried [Abstract]        
Land 482      
Buildings and Improvements 2,258      
Total 2,740      
Accumulated Depreciation at December 31, 2015 $ 1,181      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1996      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 351 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Carrying Amount at Which Carried [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Total 1,598      
Accumulated Depreciation at December 31, 2015 $ 149      
Real Estate And Accumulated Depreciation Year Of Acquisition 2000      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 150 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 11,925      
Buildings and Improvements 36,986      
Net Improvements (Retirements) Since Acquisition 12,574      
Carrying Amount at Which Carried [Abstract]        
Land 11,897      
Buildings and Improvements 49,588      
Total 61,485      
Accumulated Depreciation at December 31, 2015 $ 18,681      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | One Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,323      
Buildings and Improvements 28,613      
Net Improvements (Retirements) Since Acquisition 23,135      
Carrying Amount at Which Carried [Abstract]        
Land 7,323      
Buildings and Improvements 51,748      
Total 59,071      
Accumulated Depreciation at December 31, 2015 $ 21,189      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | 201 King of Prussia Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,956      
Buildings and Improvements 29,811      
Net Improvements (Retirements) Since Acquisition 4,530      
Carrying Amount at Which Carried [Abstract]        
Land 8,949      
Buildings and Improvements 34,348      
Total 43,297      
Accumulated Depreciation at December 31, 2015 $ 16,172      
Real Estate And Accumulated Depreciation Construction Year 2001      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 555 Lancaster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,014      
Buildings and Improvements 16,508      
Net Improvements (Retirements) Since Acquisition 16,414      
Carrying Amount at Which Carried [Abstract]        
Land 8,609      
Buildings and Improvements 32,327      
Total 40,936      
Accumulated Depreciation at December 31, 2015 $ 13,850      
Real Estate And Accumulated Depreciation Construction Year 1973      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 24 years      
PENNSYLVANIA SUBURBS [Member] | Four Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,406      
Buildings and Improvements 21,390      
Net Improvements (Retirements) Since Acquisition 14,393      
Carrying Amount at Which Carried [Abstract]        
Land 5,705      
Buildings and Improvements 35,484      
Total 41,189      
Accumulated Depreciation at December 31, 2015 $ 11,578      
Real Estate And Accumulated Depreciation Construction Year 1995      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 30 years      
PENNSYLVANIA SUBURBS [Member] | Five Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,506      
Buildings and Improvements 25,525      
Net Improvements (Retirements) Since Acquisition 5,512      
Carrying Amount at Which Carried [Abstract]        
Land 6,578      
Buildings and Improvements 30,965      
Total 37,543      
Accumulated Depreciation at December 31, 2015 $ 9,253      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 38 years      
PENNSYLVANIA SUBURBS [Member] | Three Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,773      
Buildings and Improvements 17,961      
Net Improvements (Retirements) Since Acquisition 2,540      
Carrying Amount at Which Carried [Abstract]        
Land 4,791      
Buildings and Improvements 20,483      
Total 25,274      
Accumulated Depreciation at December 31, 2015 $ 9,029      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | Two Radnor Corporate Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,937      
Buildings and Improvements 15,484      
Net Improvements (Retirements) Since Acquisition 3,883      
Carrying Amount at Which Carried [Abstract]        
Land 3,942      
Buildings and Improvements 19,362      
Total 23,304      
Accumulated Depreciation at December 31, 2015 $ 7,797      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 29 years      
PENNSYLVANIA SUBURBS [Member] | 130 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,573      
Buildings and Improvements 8,338      
Net Improvements (Retirements) Since Acquisition 3,483      
Carrying Amount at Which Carried [Abstract]        
Land 2,567      
Buildings and Improvements 11,827      
Total 14,394      
Accumulated Depreciation at December 31, 2015 $ 4,895      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 170 Radnor Chester Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,514      
Buildings and Improvements 8,147      
Net Improvements (Retirements) Since Acquisition 2,486      
Carrying Amount at Which Carried [Abstract]        
Land 2,509      
Buildings and Improvements 10,638      
Total 13,147      
Accumulated Depreciation at December 31, 2015 $ 3,488      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 25 years      
PENNSYLVANIA SUBURBS [Member] | 200 N. Radnor Chester Rd. [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,366      
Net Improvements (Retirements) Since Acquisition 3,583      
Carrying Amount at Which Carried [Abstract]        
Land 3,366      
Buildings and Improvements 3,583      
Total 6,949      
Accumulated Depreciation at December 31, 2015 $ 239      
Real Estate And Accumulated Depreciation Construction Year 2014      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 101 West Elm Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,251      
Buildings and Improvements 25,209      
Net Improvements (Retirements) Since Acquisition 3,483      
Carrying Amount at Which Carried [Abstract]        
Land 6,251      
Buildings and Improvements 28,692      
Total 34,943      
Accumulated Depreciation at December 31, 2015 $ 8,481      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
PENNSYLVANIA SUBURBS [Member] | 1 West Elm Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,557      
Buildings and Improvements 14,249      
Net Improvements (Retirements) Since Acquisition 3,069      
Carrying Amount at Which Carried [Abstract]        
Land 3,557      
Buildings and Improvements 17,318      
Total 20,875      
Accumulated Depreciation at December 31, 2015 $ 4,089      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 2970 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 177,425      
Initial Cost [Abstract]        
Land 22,430      
Buildings and Improvements 217,763      
Net Improvements (Retirements) Since Acquisition 25,597      
Carrying Amount at Which Carried [Abstract]        
Land 22,430      
Buildings and Improvements 243,360      
Total 265,790      
Accumulated Depreciation at December 31, 2015 $ 34,339      
Real Estate And Accumulated Depreciation Construction Year 2010      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 2929 Arch Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 208,570      
Net Improvements (Retirements) Since Acquisition 27,724      
Carrying Amount at Which Carried [Abstract]        
Land 12,586      
Buildings and Improvements 223,708      
Total 236,294      
Accumulated Depreciation at December 31, 2015 $ 93,087      
Real Estate And Accumulated Depreciation Construction Year 2005      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 1717 Arch Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 98,188      
Net Improvements (Retirements) Since Acquisition 68,523      
Carrying Amount at Which Carried [Abstract]        
Land 25,195      
Buildings and Improvements 141,516      
Total 166,711      
Accumulated Depreciation at December 31, 2015 $ 26,182      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2010      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 2001 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 112,000      
Initial Cost [Abstract]        
Land 15,323      
Buildings and Improvements 120,200      
Net Improvements (Retirements) Since Acquisition 17,323      
Carrying Amount at Which Carried [Abstract]        
Land 15,323      
Buildings and Improvements 137,523      
Total 152,846      
Accumulated Depreciation at December 31, 2015 $ 7,332      
Real Estate And Accumulated Depreciation Construction Year 1992      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 130 North 18th Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 14,496      
Buildings and Improvements 107,736      
Net Improvements (Retirements) Since Acquisition 27,407      
Carrying Amount at Which Carried [Abstract]        
Land 14,473      
Buildings and Improvements 135,166      
Total 149,639      
Accumulated Depreciation at December 31, 2015 $ 41,218      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 34 years      
Philadelphia CBD [Member] | 100 North 18th Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 86,886      
Initial Cost [Abstract]        
Land 16,066      
Buildings and Improvements 100,255      
Net Improvements (Retirements) Since Acquisition 12,834      
Carrying Amount at Which Carried [Abstract]        
Land 16,066      
Buildings and Improvements 113,089      
Total 129,155      
Accumulated Depreciation at December 31, 2015 $ 33,526      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 36 years      
Philadelphia CBD [Member] | 2005 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 130,000      
Initial Cost [Abstract]        
Land 15,161      
Buildings and Improvements 105,021      
Net Improvements (Retirements) Since Acquisition 14,907      
Carrying Amount at Which Carried [Abstract]        
Land 15,161      
Buildings and Improvements 119,928      
Total 135,089      
Accumulated Depreciation at December 31, 2015 $ 6,783      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2013      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Cira Centre South Garage [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 35,546      
Initial Cost [Abstract]        
Buildings and Improvements 76,008      
Net Improvements (Retirements) Since Acquisition 2,538      
Carrying Amount at Which Carried [Abstract]        
Buildings and Improvements 78,546      
Total 78,546      
Accumulated Depreciation at December 31, 2015 $ 10,467      
Real Estate And Accumulated Depreciation Construction Year 2010      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 1900 Market [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,768      
Buildings and Improvements 17,263      
Net Improvements (Retirements) Since Acquisition 17,545      
Carrying Amount at Which Carried [Abstract]        
Land 7,768      
Buildings and Improvements 34,808      
Total 42,576      
Accumulated Depreciation at December 31, 2015 $ 2,093      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 2012      
Depreciable Life (Years) 30 years      
Philadelphia CBD [Member] | 3020 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 21,417      
Net Improvements (Retirements) Since Acquisition 7,975      
Carrying Amount at Which Carried [Abstract]        
Buildings and Improvements 29,392      
Total 29,392      
Accumulated Depreciation at December 31, 2015 $ 4,685      
Real Estate And Accumulated Depreciation Construction Year 1959      
Real Estate And Accumulated Depreciation Year Of Acquisition 2011      
Depreciable Life (Years) 26 years      
Philadelphia CBD [Member] | 101-103 Juniper Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 14,401      
Net Improvements (Retirements) Since Acquisition 324      
Carrying Amount at Which Carried [Abstract]        
Land 478      
Buildings and Improvements 14,247      
Total 14,725      
Accumulated Depreciation at December 31, 2015 $ 2,390      
Real Estate And Accumulated Depreciation Construction Year 2010      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Philadelphia Marine Center [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 532      
Buildings and Improvements 2,196      
Net Improvements (Retirements) Since Acquisition 3,444      
Carrying Amount at Which Carried [Abstract]        
Land 628      
Buildings and Improvements 5,544      
Total 6,172      
Accumulated Depreciation at December 31, 2015 $ 2,679      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | 618-634 Market Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 13,365      
Buildings and Improvements 5,791      
Carrying Amount at Which Carried [Abstract]        
Land 13,365      
Buildings and Improvements 5,791      
Total 19,156      
Accumulated Depreciation at December 31, 2015 $ 869      
Real Estate And Accumulated Depreciation Construction Year 1966      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 5 years      
Metropolitan DC [Member] | 11720 Beltsville Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,831      
Buildings and Improvements 16,661      
Net Improvements (Retirements) Since Acquisition (5,279)      
Carrying Amount at Which Carried [Abstract]        
Land 1,927      
Buildings and Improvements 13,286      
Total 15,213      
Accumulated Depreciation at December 31, 2015 $ 6,581      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 11700 Beltsville Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,808      
Buildings and Improvements 12,081      
Net Improvements (Retirements) Since Acquisition (10,493)      
Carrying Amount at Which Carried [Abstract]        
Land 336      
Buildings and Improvements 4,060      
Total 4,396      
Accumulated Depreciation at December 31, 2015 $ 2,963      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 11710 Beltsville Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,278      
Buildings and Improvements 11,100      
Net Improvements (Retirements) Since Acquisition (8,534)      
Carrying Amount at Which Carried [Abstract]        
Land 534      
Buildings and Improvements 4,310      
Total 4,844      
Accumulated Depreciation at December 31, 2015 $ 2,560      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 6600 Rockledge Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 37,421      
Net Improvements (Retirements) Since Acquisition 10,749      
Carrying Amount at Which Carried [Abstract]        
Buildings and Improvements 48,170      
Total 48,170      
Accumulated Depreciation at December 31, 2015 $ 10,174      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 50 years      
Metropolitan DC [Member] | 11740 Beltsville Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 198      
Buildings and Improvements 870      
Net Improvements (Retirements) Since Acquisition 42      
Carrying Amount at Which Carried [Abstract]        
Land 202      
Buildings and Improvements 908      
Total 1,110      
Accumulated Depreciation at December 31, 2015 $ 233      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 12015 Lee Jackson Memorial Highway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,770      
Buildings and Improvements 22,895      
Net Improvements (Retirements) Since Acquisition 1,902      
Carrying Amount at Which Carried [Abstract]        
Land 3,561      
Buildings and Improvements 25,006      
Total 28,567      
Accumulated Depreciation at December 31, 2015 $ 7,574      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 42 years      
Metropolitan DC [Member] | 11781 Lee Jackson Memorial Highway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,246      
Buildings and Improvements 19,836      
Net Improvements (Retirements) Since Acquisition (1,435)      
Carrying Amount at Which Carried [Abstract]        
Land 2,615      
Buildings and Improvements 19,032      
Total 21,647      
Accumulated Depreciation at December 31, 2015 $ 5,512      
Real Estate And Accumulated Depreciation Construction Year 1982      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 4401 Fair Lakes Court [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,569      
Buildings and Improvements 11,982      
Net Improvements (Retirements) Since Acquisition (2,098)      
Carrying Amount at Which Carried [Abstract]        
Land 990      
Buildings and Improvements 10,463      
Total 11,453      
Accumulated Depreciation at December 31, 2015 $ 2,766      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 52 years      
Metropolitan DC [Member] | 3141 Fairview Park Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 20,838      
Initial Cost [Abstract]        
Land 5,918      
Buildings and Improvements 40,981      
Net Improvements (Retirements) Since Acquisition 14,119      
Carrying Amount at Which Carried [Abstract]        
Land 7,081      
Buildings and Improvements 53,937      
Total 61,018      
Accumulated Depreciation at December 31, 2015 $ 11,640      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 51 years      
Metropolitan DC [Member] | 2340 Dulles Corner Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 16,345      
Buildings and Improvements 65,379      
Net Improvements (Retirements) Since Acquisition 18,280      
Carrying Amount at Which Carried [Abstract]        
Land 16,129      
Buildings and Improvements 83,875      
Total 100,004      
Accumulated Depreciation at December 31, 2015 $ 31,570      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2291 Wood Oak Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 8,243      
Buildings and Improvements 52,413      
Net Improvements (Retirements) Since Acquisition 12,524      
Carrying Amount at Which Carried [Abstract]        
Land 8,782      
Buildings and Improvements 64,398      
Total 73,180      
Accumulated Depreciation at December 31, 2015 $ 15,294      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 196/198 Van Buren Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,931      
Buildings and Improvements 43,812      
Net Improvements (Retirements) Since Acquisition 9,059      
Carrying Amount at Which Carried [Abstract]        
Land 8,348      
Buildings and Improvements 52,454      
Total 60,802      
Accumulated Depreciation at December 31, 2015 $ 11,986      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 53 years      
Metropolitan DC [Member] | 2251 Corporate Park Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 11,472      
Buildings and Improvements 45,893      
Net Improvements (Retirements) Since Acquisition 3,367      
Carrying Amount at Which Carried [Abstract]        
Land 11,472      
Buildings and Improvements 49,260      
Total 60,732      
Accumulated Depreciation at December 31, 2015 $ 11,204      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2355 Dulles Corner Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 10,365      
Buildings and Improvements 43,876      
Net Improvements (Retirements) Since Acquisition 4,612      
Carrying Amount at Which Carried [Abstract]        
Land 10,365      
Buildings and Improvements 48,488      
Total 58,853      
Accumulated Depreciation at December 31, 2015 $ 13,849      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Metropolitan DC [Member] | 2411 Dulles Corner Park [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,279      
Buildings and Improvements 46,340      
Net Improvements (Retirements) Since Acquisition 16,028      
Carrying Amount at Which Carried [Abstract]        
Land 7,417      
Buildings and Improvements 62,230      
Total 69,647      
Accumulated Depreciation at December 31, 2015 $ 10,822      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 50 years      
Metropolitan DC [Member] | 13880 Dulles Corner Lane [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,236      
Buildings and Improvements 39,213      
Net Improvements (Retirements) Since Acquisition 4,556      
Carrying Amount at Which Carried [Abstract]        
Land 7,373      
Buildings and Improvements 43,632      
Total 51,005      
Accumulated Depreciation at December 31, 2015 $ 9,193      
Real Estate And Accumulated Depreciation Construction Year 1997      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 2121 Cooperative Way [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,598      
Buildings and Improvements 38,639      
Net Improvements (Retirements) Since Acquisition 2,688      
Carrying Amount at Which Carried [Abstract]        
Land 5,795      
Buildings and Improvements 41,130      
Total 46,925      
Accumulated Depreciation at December 31, 2015 $ 8,879      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 54 years      
Metropolitan DC [Member] | 2201 Cooperative Way [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,809      
Buildings and Improvements 34,093      
Net Improvements (Retirements) Since Acquisition 5,317      
Carrying Amount at Which Carried [Abstract]        
Land 4,809      
Buildings and Improvements 39,410      
Total 44,219      
Accumulated Depreciation at December 31, 2015 $ 8,573      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 54 years      
Metropolitan DC [Member] | 13825 Sunrise Valley Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,794      
Buildings and Improvements 19,365      
Net Improvements (Retirements) Since Acquisition 2,126      
Carrying Amount at Which Carried [Abstract]        
Land 3,866      
Buildings and Improvements 21,419      
Total 25,285      
Accumulated Depreciation at December 31, 2015 $ 5,398      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 46 years      
Metropolitan DC [Member] | 1676 International Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 18,437      
Buildings and Improvements 97,538      
Net Improvements (Retirements) Since Acquisition 3,740      
Carrying Amount at Which Carried [Abstract]        
Land 18,785      
Buildings and Improvements 100,930      
Total 119,715      
Accumulated Depreciation at December 31, 2015 $ 19,257      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 55 years      
Metropolitan DC [Member] | 8260 Greensboro Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,952      
Buildings and Improvements 33,964      
Net Improvements (Retirements) Since Acquisition 2,227      
Carrying Amount at Which Carried [Abstract]        
Land 8,102      
Buildings and Improvements 36,041      
Total 44,143      
Accumulated Depreciation at December 31, 2015 $ 7,898      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 52 years      
Metropolitan DC [Member] | 2273 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,167      
Buildings and Improvements 31,110      
Net Improvements (Retirements) Since Acquisition 4,638      
Carrying Amount at Which Carried [Abstract]        
Land 5,237      
Buildings and Improvements 35,678      
Total 40,915      
Accumulated Depreciation at December 31, 2015 $ 10,292      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 2275 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,059      
Buildings and Improvements 29,668      
Net Improvements (Retirements) Since Acquisition 7,413      
Carrying Amount at Which Carried [Abstract]        
Land 5,154      
Buildings and Improvements 36,986      
Total 42,140      
Accumulated Depreciation at December 31, 2015 $ 9,057      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 2277 Research Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,649      
Buildings and Improvements 26,952      
Net Improvements (Retirements) Since Acquisition 17,817      
Carrying Amount at Which Carried [Abstract]        
Land 4,733      
Buildings and Improvements 44,685      
Total 49,418      
Accumulated Depreciation at December 31, 2015 $ 7,715      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 45 years      
Metropolitan DC [Member] | 1900 Gallows Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 7,797      
Buildings and Improvements 47,817      
Net Improvements (Retirements) Since Acquisition 11,469      
Carrying Amount at Which Carried [Abstract]        
Land 7,944      
Buildings and Improvements 59,139      
Total 67,083      
Accumulated Depreciation at December 31, 2015 $ 13,191      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 52 years      
Metropolitan DC [Member] | 8521 Leesburg Pike [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,316      
Buildings and Improvements 30,885      
Net Improvements (Retirements) Since Acquisition 5,280      
Carrying Amount at Which Carried [Abstract]        
Land 4,397      
Buildings and Improvements 36,084      
Total 40,481      
Accumulated Depreciation at December 31, 2015 $ 7,393      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 51 years      
New Jersey/ Delaware [Member] | 200 Lake Drive East [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,069      
Buildings and Improvements 8,275      
Net Improvements (Retirements) Since Acquisition 1,798      
Carrying Amount at Which Carried [Abstract]        
Land 2,069      
Buildings and Improvements 10,073      
Total 12,142      
Accumulated Depreciation at December 31, 2015 $ 3,922      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 220 Lake Drive East [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,144      
Buildings and Improvements 8,798      
Net Improvements (Retirements) Since Acquisition 1,955      
Carrying Amount at Which Carried [Abstract]        
Land 2,144      
Buildings and Improvements 10,753      
Total 12,897      
Accumulated Depreciation at December 31, 2015 $ 3,818      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 210 Lake Drive East [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,645      
Buildings and Improvements 6,579      
Net Improvements (Retirements) Since Acquisition 2,748      
Carrying Amount at Which Carried [Abstract]        
Land 1,645      
Buildings and Improvements 9,327      
Total 10,972      
Accumulated Depreciation at December 31, 2015 $ 3,102      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 20 East Clementon Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 769      
Buildings and Improvements 3,055      
Net Improvements (Retirements) Since Acquisition 401      
Carrying Amount at Which Carried [Abstract]        
Land 769      
Buildings and Improvements 3,456      
Total 4,225      
Accumulated Depreciation at December 31, 2015 $ 1,712      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 10 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 244      
Buildings and Improvements 971      
Net Improvements (Retirements) Since Acquisition 78      
Carrying Amount at Which Carried [Abstract]        
Land 244      
Buildings and Improvements 1,049      
Total 1,293      
Accumulated Depreciation at December 31, 2015 $ 525      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 7 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 231      
Buildings and Improvements 921      
Net Improvements (Retirements) Since Acquisition 123      
Carrying Amount at Which Carried [Abstract]        
Land 231      
Buildings and Improvements 1,044      
Total 1,275      
Accumulated Depreciation at December 31, 2015 $ 514      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 50 East Clementon Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 114      
Buildings and Improvements 964      
Net Improvements (Retirements) Since Acquisition 671      
Carrying Amount at Which Carried [Abstract]        
Land 114      
Buildings and Improvements 1,635      
Total 1,749      
Accumulated Depreciation at December 31, 2015 $ 584      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 2 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 185      
Buildings and Improvements 730      
Net Improvements (Retirements) Since Acquisition 16      
Carrying Amount at Which Carried [Abstract]        
Land 185      
Buildings and Improvements 746      
Total 931      
Accumulated Depreciation at December 31, 2015 $ 369      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 4 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 183      
Buildings and Improvements 726      
Net Improvements (Retirements) Since Acquisition 16      
Carrying Amount at Which Carried [Abstract]        
Land 183      
Buildings and Improvements 742      
Total 925      
Accumulated Depreciation at December 31, 2015 $ 362      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 1 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 93      
Buildings and Improvements 364      
Net Improvements (Retirements) Since Acquisition 57      
Carrying Amount at Which Carried [Abstract]        
Land 93      
Buildings and Improvements 421      
Total 514      
Accumulated Depreciation at December 31, 2015 $ 228      
Real Estate And Accumulated Depreciation Construction Year 1972      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 5 U.S. Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 21      
Buildings and Improvements 81      
Net Improvements (Retirements) Since Acquisition 3      
Carrying Amount at Which Carried [Abstract]        
Land 21      
Buildings and Improvements 84      
Total 105      
Accumulated Depreciation at December 31, 2015 $ 41      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 5 Foster Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 9      
Buildings and Improvements 32      
Net Improvements (Retirements) Since Acquisition 26      
Carrying Amount at Which Carried [Abstract]        
Land 9      
Buildings and Improvements 58      
Total 67      
Accumulated Depreciation at December 31, 2015 $ 30      
Real Estate And Accumulated Depreciation Construction Year 1968      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Two Eves Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 818      
Buildings and Improvements 3,461      
Net Improvements (Retirements) Since Acquisition 269      
Carrying Amount at Which Carried [Abstract]        
Land 818      
Buildings and Improvements 3,730      
Total 4,548      
Accumulated Depreciation at December 31, 2015 $ 1,761      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Five Eves Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 703      
Buildings and Improvements 2,819      
Net Improvements (Retirements) Since Acquisition 1,437      
Carrying Amount at Which Carried [Abstract]        
Land 703      
Buildings and Improvements 4,256      
Total 4,959      
Accumulated Depreciation at December 31, 2015 $ 1,760      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Four B Eves Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 588      
Buildings and Improvements 2,369      
Net Improvements (Retirements) Since Acquisition 185      
Carrying Amount at Which Carried [Abstract]        
Land 588      
Buildings and Improvements 2,554      
Total 3,142      
Accumulated Depreciation at December 31, 2015 $ 1,311      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Four A Eves Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 539      
Buildings and Improvements 2,168      
Net Improvements (Retirements) Since Acquisition 376      
Carrying Amount at Which Carried [Abstract]        
Land 539      
Buildings and Improvements 2,544      
Total 3,083      
Accumulated Depreciation at December 31, 2015 $ 1,215      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 308 Harper Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,643      
Buildings and Improvements 6,663      
Net Improvements (Retirements) Since Acquisition (39)      
Carrying Amount at Which Carried [Abstract]        
Land 1,425      
Buildings and Improvements 6,842      
Total 8,267      
Accumulated Depreciation at December 31, 2015 $ 3,481      
Real Estate And Accumulated Depreciation Construction Year 1976      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 1120 Executive Boulevard [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,074      
Buildings and Improvements 8,415      
Net Improvements (Retirements) Since Acquisition 4,280      
Carrying Amount at Which Carried [Abstract]        
Land 2,074      
Buildings and Improvements 12,695      
Total 14,769      
Accumulated Depreciation at December 31, 2015 $ 5,696      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 700 East Gate Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,569      
Buildings and Improvements 14,436      
Net Improvements (Retirements) Since Acquisition 1,956      
Carrying Amount at Which Carried [Abstract]        
Land 3,569      
Buildings and Improvements 16,392      
Total 19,961      
Accumulated Depreciation at December 31, 2015 $ 7,573      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 701 East Gate Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,736      
Buildings and Improvements 6,877      
Net Improvements (Retirements) Since Acquisition 1,163      
Carrying Amount at Which Carried [Abstract]        
Land 1,589      
Buildings and Improvements 8,187      
Total 9,776      
Accumulated Depreciation at December 31, 2015 $ 4,033      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 307 Fellowship Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,565      
Buildings and Improvements 6,342      
Net Improvements (Retirements) Since Acquisition 306      
Carrying Amount at Which Carried [Abstract]        
Land 1,330      
Buildings and Improvements 6,883      
Total 8,213      
Accumulated Depreciation at December 31, 2015 $ 3,551      
Real Estate And Accumulated Depreciation Construction Year 1981      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 305 Fellowship Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,421      
Buildings and Improvements 5,768      
Net Improvements (Retirements) Since Acquisition 920      
Carrying Amount at Which Carried [Abstract]        
Land 1,233      
Buildings and Improvements 6,876      
Total 8,109      
Accumulated Depreciation at December 31, 2015 $ 3,340      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 303 Fellowship Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,493      
Buildings and Improvements 6,055      
Net Improvements (Retirements) Since Acquisition 205      
Carrying Amount at Which Carried [Abstract]        
Land 1,250      
Buildings and Improvements 6,503      
Total 7,753      
Accumulated Depreciation at December 31, 2015 $ 3,213      
Real Estate And Accumulated Depreciation Construction Year 1979      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 309 Fellowship Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,518      
Buildings and Improvements 6,154      
Net Improvements (Retirements) Since Acquisition 711      
Carrying Amount at Which Carried [Abstract]        
Land 1,518      
Buildings and Improvements 6,865      
Total 8,383      
Accumulated Depreciation at December 31, 2015 $ 3,206      
Real Estate And Accumulated Depreciation Construction Year 1982      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 161 Gaither Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,016      
Buildings and Improvements 4,064      
Net Improvements (Retirements) Since Acquisition 535      
Carrying Amount at Which Carried [Abstract]        
Land 1,011      
Buildings and Improvements 4,604      
Total 5,615      
Accumulated Depreciation at December 31, 2015 $ 1,724      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 815 East Gate Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 636      
Buildings and Improvements 2,584      
Net Improvements (Retirements) Since Acquisition 446      
Carrying Amount at Which Carried [Abstract]        
Land 581      
Buildings and Improvements 3,085      
Total 3,666      
Accumulated Depreciation at December 31, 2015 $ 1,532      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 817 East Gate Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 611      
Buildings and Improvements 2,426      
Net Improvements (Retirements) Since Acquisition 216      
Carrying Amount at Which Carried [Abstract]        
Land 600      
Buildings and Improvements 2,653      
Total 3,253      
Accumulated Depreciation at December 31, 2015 $ 1,247      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Main Street-Plaza 1000 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,732      
Buildings and Improvements 10,942      
Net Improvements (Retirements) Since Acquisition 96      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 11,038      
Total 13,770      
Accumulated Depreciation at December 31, 2015 $ 11,002      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Main Street-Piazza [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 696      
Buildings and Improvements 2,802      
Net Improvements (Retirements) Since Acquisition 3,360      
Carrying Amount at Which Carried [Abstract]        
Land 696      
Buildings and Improvements 6,162      
Total 6,858      
Accumulated Depreciation at December 31, 2015 $ 2,035      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | Main Street-Promenade [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 532      
Buildings and Improvements 2,052      
Net Improvements (Retirements) Since Acquisition 538      
Carrying Amount at Which Carried [Abstract]        
Land 532      
Buildings and Improvements 2,590      
Total 3,122      
Accumulated Depreciation at December 31, 2015 $ 1,288      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
New Jersey/ Delaware [Member] | 920 North King Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,141      
Buildings and Improvements 21,140      
Net Improvements (Retirements) Since Acquisition 2,707      
Carrying Amount at Which Carried [Abstract]        
Land 6,141      
Buildings and Improvements 23,847      
Total 29,988      
Accumulated Depreciation at December 31, 2015 $ 8,113      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 30 years      
New Jersey/ Delaware [Member] | 300 Delaware Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,369      
Buildings and Improvements 13,739      
Net Improvements (Retirements) Since Acquisition 2,889      
Carrying Amount at Which Carried [Abstract]        
Land 6,369      
Buildings and Improvements 16,628      
Total 22,997      
Accumulated Depreciation at December 31, 2015 $ 7,496      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 23 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 1 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,755      
Buildings and Improvements 22,702      
Net Improvements (Retirements) Since Acquisition 119      
Carrying Amount at Which Carried [Abstract]        
Land 3,755      
Buildings and Improvements 22,821      
Total 26,576      
Accumulated Depreciation at December 31, 2015 $ 358      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 2 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,732      
Buildings and Improvements 16,305      
Net Improvements (Retirements) Since Acquisition 654      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 16,959      
Total 19,691      
Accumulated Depreciation at December 31, 2015 $ 274      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 3 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,688      
Buildings and Improvements 22,348      
Net Improvements (Retirements) Since Acquisition 118      
Carrying Amount at Which Carried [Abstract]        
Land 3,688      
Buildings and Improvements 22,466      
Total 26,154      
Accumulated Depreciation at December 31, 2015 $ 353      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 4 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,614      
Buildings and Improvements 15,740      
Net Improvements (Retirements) Since Acquisition 83      
Carrying Amount at Which Carried [Abstract]        
Land 2,614      
Buildings and Improvements 15,823      
Total 18,437      
Accumulated Depreciation at December 31, 2015 $ 248      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 5 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 3,689      
Buildings and Improvements 22,354      
Net Improvements (Retirements) Since Acquisition 117      
Carrying Amount at Which Carried [Abstract]        
Land 3,689      
Buildings and Improvements 22,471      
Total 26,160      
Accumulated Depreciation at December 31, 2015 $ 353      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 6 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,676      
Buildings and Improvements 15,972      
Net Improvements (Retirements) Since Acquisition 84      
Carrying Amount at Which Carried [Abstract]        
Land 2,676      
Buildings and Improvements 16,056      
Total 18,732      
Accumulated Depreciation at December 31, 2015 $ 252      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Building 8 [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,400      
Buildings and Improvements 7,422      
Net Improvements (Retirements) Since Acquisition 39      
Carrying Amount at Which Carried [Abstract]        
Land 1,400      
Buildings and Improvements 7,461      
Total 8,861      
Accumulated Depreciation at December 31, 2015 $ 122      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Austin, Texas [Member] | 11501 Burnet Road - Parking Garage [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 19,826      
Net Improvements (Retirements) Since Acquisition 103      
Carrying Amount at Which Carried [Abstract]        
Buildings and Improvements 19,929      
Total 19,929      
Accumulated Depreciation at December 31, 2015 $ 424      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Richmond, Virginia [Member] | 4364 South Alston Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,622      
Buildings and Improvements 6,419      
Net Improvements (Retirements) Since Acquisition (454)      
Carrying Amount at Which Carried [Abstract]        
Land 1,144      
Buildings and Improvements 6,443      
Total 7,587      
Accumulated Depreciation at December 31, 2015 $ 3,512      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 4805 Lake Brooke Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,640      
Buildings and Improvements 6,567      
Net Improvements (Retirements) Since Acquisition 703      
Carrying Amount at Which Carried [Abstract]        
Land 1,420      
Buildings and Improvements 7,490      
Total 8,910      
Accumulated Depreciation at December 31, 2015 $ 3,576      
Real Estate And Accumulated Depreciation Construction Year 1996      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | Overlook I [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 748      
Buildings and Improvements 3,976      
Net Improvements (Retirements) Since Acquisition 600      
Carrying Amount at Which Carried [Abstract]        
Land 791      
Buildings and Improvements 4,533      
Total 5,324      
Accumulated Depreciation at December 31, 2015 $ 916      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2011      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | Overlook II [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 748      
Buildings and Improvements 4,492      
Net Improvements (Retirements) Since Acquisition 504      
Carrying Amount at Which Carried [Abstract]        
Land 791      
Buildings and Improvements 4,953      
Total 5,744      
Accumulated Depreciation at December 31, 2015 $ 923      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2011      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2812 Emerywood Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,069      
Buildings and Improvements 4,281      
Net Improvements (Retirements) Since Acquisition 670      
Carrying Amount at Which Carried [Abstract]        
Land 1,069      
Buildings and Improvements 4,951      
Total 6,020      
Accumulated Depreciation at December 31, 2015 $ 2,290      
Real Estate And Accumulated Depreciation Construction Year 1980      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 300 Arboretum Place [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 5,450      
Buildings and Improvements 21,892      
Net Improvements (Retirements) Since Acquisition 3,831      
Carrying Amount at Which Carried [Abstract]        
Land 4,674      
Buildings and Improvements 26,499      
Total 31,173      
Accumulated Depreciation at December 31, 2015 $ 12,104      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 7501 Boulders View Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,669      
Buildings and Improvements 19,699      
Net Improvements (Retirements) Since Acquisition (2,770)      
Carrying Amount at Which Carried [Abstract]        
Land 3,920      
Buildings and Improvements 17,678      
Total 21,598      
Accumulated Depreciation at December 31, 2015 $ 4,891      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 7300 Beaufont Springs Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,672      
Buildings and Improvements 19,689      
Net Improvements (Retirements) Since Acquisition (1,080)      
Carrying Amount at Which Carried [Abstract]        
Land 3,980      
Buildings and Improvements 19,301      
Total 23,281      
Accumulated Depreciation at December 31, 2015 $ 4,999      
Real Estate And Accumulated Depreciation Construction Year 2000      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 6800 Paragon Place [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,552      
Buildings and Improvements 18,414      
Net Improvements (Retirements) Since Acquisition (681)      
Carrying Amount at Which Carried [Abstract]        
Land 3,821      
Buildings and Improvements 18,464      
Total 22,285      
Accumulated Depreciation at December 31, 2015 $ 5,456      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 6802 Paragon Place [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,917      
Buildings and Improvements 11,454      
Net Improvements (Retirements) Since Acquisition 3,074      
Carrying Amount at Which Carried [Abstract]        
Land 2,917      
Buildings and Improvements 14,528      
Total 17,445      
Accumulated Depreciation at December 31, 2015 $ 5,706      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2002      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 1025 Boulders Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,574      
Buildings and Improvements 11,297      
Net Improvements (Retirements) Since Acquisition (1,314)      
Carrying Amount at Which Carried [Abstract]        
Land 2,040      
Buildings and Improvements 10,517      
Total 12,557      
Accumulated Depreciation at December 31, 2015 $ 3,009      
Real Estate And Accumulated Depreciation Construction Year 1994      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2100-2116 West Laburnam Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,482      
Buildings and Improvements 8,846      
Net Improvements (Retirements) Since Acquisition 2,742      
Carrying Amount at Which Carried [Abstract]        
Land 2,482      
Buildings and Improvements 11,588      
Total 14,070      
Accumulated Depreciation at December 31, 2015 $ 5,331      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 7401 Beaufont Springs Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,349      
Buildings and Improvements 10,396      
Net Improvements (Retirements) Since Acquisition (2,048)      
Carrying Amount at Which Carried [Abstract]        
Land 1,788      
Buildings and Improvements 8,909      
Total 10,697      
Accumulated Depreciation at December 31, 2015 $ 2,649      
Real Estate And Accumulated Depreciation Construction Year 1998      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 7325 Beaufont Springs Drive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,344      
Buildings and Improvements 10,377      
Net Improvements (Retirements) Since Acquisition (1,745)      
Carrying Amount at Which Carried [Abstract]        
Land 2,002      
Buildings and Improvements 8,974      
Total 10,976      
Accumulated Depreciation at December 31, 2015 $ 2,349      
Real Estate And Accumulated Depreciation Construction Year 1999      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 9011 Arboretum Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,857      
Buildings and Improvements 7,702      
Net Improvements (Retirements) Since Acquisition (430)      
Carrying Amount at Which Carried [Abstract]        
Land 1,501      
Buildings and Improvements 7,628      
Total 9,129      
Accumulated Depreciation at December 31, 2015 $ 4,104      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 6806 Paragon Place [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Buildings and Improvements 10,288      
Net Improvements (Retirements) Since Acquisition (117)      
Carrying Amount at Which Carried [Abstract]        
Land 403      
Buildings and Improvements 9,768      
Total 10,171      
Accumulated Depreciation at December 31, 2015 $ 2,885      
Real Estate And Accumulated Depreciation Construction Year 2007      
Real Estate And Accumulated Depreciation Year Of Acquisition 2005      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 9100 Arboretum Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,362      
Buildings and Improvements 5,489      
Net Improvements (Retirements) Since Acquisition 541      
Carrying Amount at Which Carried [Abstract]        
Land 1,274      
Buildings and Improvements 6,118      
Total 7,392      
Accumulated Depreciation at December 31, 2015 $ 2,974      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2511 Brittons Hill Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,202      
Buildings and Improvements 4,820      
Net Improvements (Retirements) Since Acquisition 1,221      
Carrying Amount at Which Carried [Abstract]        
Land 1,202      
Buildings and Improvements 6,041      
Total 7,243      
Accumulated Depreciation at December 31, 2015 $ 3,041      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 9200 Arboretum Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 985      
Buildings and Improvements 3,973      
Net Improvements (Retirements) Since Acquisition 1,275      
Carrying Amount at Which Carried [Abstract]        
Land 985      
Buildings and Improvements 5,248      
Total 6,233      
Accumulated Depreciation at December 31, 2015 $ 2,562      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 9210 Arboretum Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,110      
Buildings and Improvements 4,474      
Net Improvements (Retirements) Since Acquisition 167      
Carrying Amount at Which Carried [Abstract]        
Land 970      
Buildings and Improvements 4,781      
Total 5,751      
Accumulated Depreciation at December 31, 2015 $ 2,274      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2201-2245 Tomlynn Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,020      
Buildings and Improvements 4,067      
Net Improvements (Retirements) Since Acquisition 1,126      
Carrying Amount at Which Carried [Abstract]        
Land 1,020      
Buildings and Improvements 5,193      
Total 6,213      
Accumulated Depreciation at December 31, 2015 $ 2,332      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 9211 Arboretum Parkway [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 582      
Buildings and Improvements 2,433      
Net Improvements (Retirements) Since Acquisition 317      
Carrying Amount at Which Carried [Abstract]        
Land 470      
Buildings and Improvements 2,862      
Total 3,332      
Accumulated Depreciation at December 31, 2015 $ 1,395      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2221-2245 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 530      
Buildings and Improvements 2,123      
Net Improvements (Retirements) Since Acquisition 369      
Carrying Amount at Which Carried [Abstract]        
Land 530      
Buildings and Improvements 2,492      
Total 3,022      
Accumulated Depreciation at December 31, 2015 $ 1,091      
Real Estate And Accumulated Depreciation Construction Year 1994      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2212-2224 Tomlynn Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 502      
Buildings and Improvements 2,014      
Net Improvements (Retirements) Since Acquisition 487      
Carrying Amount at Which Carried [Abstract]        
Land 502      
Buildings and Improvements 2,501      
Total 3,003      
Accumulated Depreciation at December 31, 2015 $ 1,109      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2244 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 550      
Buildings and Improvements 2,203      
Carrying Amount at Which Carried [Abstract]        
Land 550      
Buildings and Improvements 2,203      
Total 2,753      
Accumulated Depreciation at December 31, 2015 $ 1,016      
Real Estate And Accumulated Depreciation Construction Year 1993      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2277 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 507      
Buildings and Improvements 2,034      
Net Improvements (Retirements) Since Acquisition 294      
Carrying Amount at Which Carried [Abstract]        
Land 507      
Buildings and Improvements 2,328      
Total 2,835      
Accumulated Depreciation at December 31, 2015 $ 1,077      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2248 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 512      
Buildings and Improvements 2,049      
Net Improvements (Retirements) Since Acquisition 379      
Carrying Amount at Which Carried [Abstract]        
Land 503      
Buildings and Improvements 2,437      
Total 2,940      
Accumulated Depreciation at December 31, 2015 $ 1,000      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2161-2179 Tomlynn Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 423      
Buildings and Improvements 1,695      
Net Improvements (Retirements) Since Acquisition 306      
Carrying Amount at Which Carried [Abstract]        
Land 423      
Buildings and Improvements 2,001      
Total 2,424      
Accumulated Depreciation at December 31, 2015 $ 867      
Real Estate And Accumulated Depreciation Construction Year 1985      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2246 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 455      
Buildings and Improvements 1,822      
Carrying Amount at Which Carried [Abstract]        
Land 455      
Buildings and Improvements 1,822      
Total 2,277      
Accumulated Depreciation at December 31, 2015 $ 841      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2251 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 387      
Buildings and Improvements 1,552      
Net Improvements (Retirements) Since Acquisition 59      
Carrying Amount at Which Carried [Abstract]        
Land 387      
Buildings and Improvements 1,611      
Total 1,998      
Accumulated Depreciation at December 31, 2015 $ 740      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2256 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 356      
Buildings and Improvements 1,427      
Net Improvements (Retirements) Since Acquisition 225      
Carrying Amount at Which Carried [Abstract]        
Land 356      
Buildings and Improvements 1,652      
Total 2,008      
Accumulated Depreciation at December 31, 2015 $ 777      
Real Estate And Accumulated Depreciation Construction Year 1982      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2130-2146 Tomlynn Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 353      
Buildings and Improvements 1,416      
Net Improvements (Retirements) Since Acquisition 225      
Carrying Amount at Which Carried [Abstract]        
Land 353      
Buildings and Improvements 1,641      
Total 1,994      
Accumulated Depreciation at December 31, 2015 $ 779      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2112-2124 Tomlynn Street [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 281      
Buildings and Improvements 1,125      
Net Improvements (Retirements) Since Acquisition 306      
Carrying Amount at Which Carried [Abstract]        
Land 281      
Buildings and Improvements 1,431      
Total 1,712      
Accumulated Depreciation at December 31, 2015 $ 695      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | 2240 Dabney Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 264      
Buildings and Improvements 1,059      
Net Improvements (Retirements) Since Acquisition 88      
Carrying Amount at Which Carried [Abstract]        
Land 264      
Buildings and Improvements 1,147      
Total 1,411      
Accumulated Depreciation at December 31, 2015 $ 514      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 1998      
Depreciable Life (Years) 40 years      
Richmond, Virginia [Member] | Boulders Land [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,256      
Net Improvements (Retirements) Since Acquisition (116)      
Carrying Amount at Which Carried [Abstract]        
Land 1,115      
Buildings and Improvements 25      
Total 1,140      
Accumulated Depreciation at December 31, 2015 $ 3      
Real Estate And Accumulated Depreciation Year Of Acquisition 2007      
California [Member] | 1200 Concord Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,395      
Buildings and Improvements 24,664      
Net Improvements (Retirements) Since Acquisition 833      
Carrying Amount at Which Carried [Abstract]        
Land 6,515      
Buildings and Improvements 25,377      
Total 31,892      
Accumulated Depreciation at December 31, 2015 $ 9,192      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 34 years      
California [Member] | 1220 Concord Avenue [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 6,476      
Buildings and Improvements 24,966      
Net Improvements (Retirements) Since Acquisition 298      
Carrying Amount at Which Carried [Abstract]        
Land 6,476      
Buildings and Improvements 25,264      
Total 31,740      
Accumulated Depreciation at December 31, 2015 $ 8,869      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 34 years      
California [Member] | Oakland Lot B [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 4,342      
Net Improvements (Retirements) Since Acquisition 17      
Carrying Amount at Which Carried [Abstract]        
Land 4,332      
Buildings and Improvements 27      
Total $ 4,359      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006