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Schedule III - Real Estate and Accumulated Depreciation (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 325,038      
Initial Cost [Abstract]        
Land 450,881      
Buildings and Improvements 2,669,016      
Net Improvements (Retirements) Since Acquisition 539,989      
Carrying Amount at Which Carried [Abstract]        
Land 481,282      
Buildings and Improvements 3,178,604      
Total 3,659,886 $ 4,487,588 $ 4,631,128 $ 4,669,289
Accumulated Depreciation at December 31, 2016 885,392 $ 1,080,616 $ 1,078,996 $ 983,808
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, 2016 $ 7,132      
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 4,014      
Carrying Amount at Which Carried [Abstract]        
Land 2,206      
Buildings and Improvements 17,436      
Total 19,642      
Accumulated Depreciation at December 31, 2016 $ 9,213      
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 6,130      
Carrying Amount at Which Carried [Abstract]        
Land 2,611      
Buildings and Improvements 16,575      
Total 19,186      
Accumulated Depreciation at December 31, 2016 $ 9,450      
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 2,996      
Carrying Amount at Which Carried [Abstract]        
Land 2,595      
Buildings and Improvements 14,805      
Total 17,400      
Accumulated Depreciation at December 31, 2016 $ 8,043      
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 1,864      
Carrying Amount at Which Carried [Abstract]        
Land 1,533      
Buildings and Improvements 11,324      
Total 12,857      
Accumulated Depreciation at December 31, 2016 $ 5,947      
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 3,110      
Carrying Amount at Which Carried [Abstract]        
Land 2,086      
Buildings and Improvements 11,452      
Total 13,538      
Accumulated Depreciation at December 31, 2016 $ 4,429      
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 1,483      
Carrying Amount at Which Carried [Abstract]        
Land 1,180      
Buildings and Improvements 8,773      
Total 9,953      
Accumulated Depreciation at December 31, 2016 $ 4,508      
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, 2016 $ 4,437      
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 1,271      
Carrying Amount at Which Carried [Abstract]        
Land 1,695      
Buildings and Improvements 8,050      
Total 9,745      
Accumulated Depreciation at December 31, 2016 $ 3,133      
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] | Six Tower Bridge (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 1,504      
Carrying Amount at Which Carried [Abstract]        
Land 6,237      
Buildings and Improvements 16,916      
Total 23,153      
Accumulated Depreciation at December 31, 2016 $ 1,838      
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, 2016 $ 3,812      
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 3,852      
Carrying Amount at Which Carried [Abstract]        
Land 4,241      
Buildings and Improvements 20,431      
Total 24,672      
Accumulated Depreciation at December 31, 2016 $ 8,834      
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] | 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 5,060      
Carrying Amount at Which Carried [Abstract]        
Land 4,222      
Buildings and Improvements 21,951      
Total 26,173      
Accumulated Depreciation at December 31, 2016 $ 10,997      
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] | 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 12,614      
Carrying Amount at Which Carried [Abstract]        
Land 2,894      
Buildings and Improvements 16,584      
Total 19,478      
Accumulated Depreciation at December 31, 2016 $ 6,597      
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 2,115      
Carrying Amount at Which Carried [Abstract]        
Land 2,770      
Buildings and Improvements 13,129      
Total 15,899      
Accumulated Depreciation at December 31, 2016 $ 7,064      
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 2,495      
Carrying Amount at Which Carried [Abstract]        
Land 2,772      
Buildings and Improvements 13,431      
Total 16,203      
Accumulated Depreciation at December 31, 2016 $ 6,427      
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 4,261      
Carrying Amount at Which Carried [Abstract]        
Land 2,712      
Buildings and Improvements 15,214      
Total 17,926      
Accumulated Depreciation at December 31, 2016 $ 6,705      
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 3,786      
Carrying Amount at Which Carried [Abstract]        
Land 2,773      
Buildings and Improvements 14,930      
Total 17,703      
Accumulated Depreciation at December 31, 2016 $ 7,019      
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, 2016 $ 8,469      
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, 2016 $ 6,699      
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 2,878      
Carrying Amount at Which Carried [Abstract]        
Land 2,017      
Buildings and Improvements 10,948      
Total 12,965      
Accumulated Depreciation at December 31, 2016 $ 5,174      
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 1,561      
Carrying Amount at Which Carried [Abstract]        
Land 1,916      
Buildings and Improvements 5,939      
Total 7,855      
Accumulated Depreciation at December 31, 2016 $ 3,295      
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] | 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, 2016 $ 2,286      
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] | 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, 2016 $ 4,008      
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 (924)      
Carrying Amount at Which Carried [Abstract]        
Land 1,108      
Buildings and Improvements 4,231      
Total 5,339      
Accumulated Depreciation at December 31, 2016 $ 1,595      
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 998      
Carrying Amount at Which Carried [Abstract]        
Land 1,112      
Buildings and Improvements 5,065      
Total 6,177      
Accumulated Depreciation at December 31, 2016 $ 2,286      
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 285      
Carrying Amount at Which Carried [Abstract]        
Land 1,164      
Buildings and Improvements 4,181      
Total 5,345      
Accumulated Depreciation at December 31, 2016 $ 1,485      
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 856      
Carrying Amount at Which Carried [Abstract]        
Land 787      
Buildings and Improvements 4,168      
Total 4,955      
Accumulated Depreciation at December 31, 2016 $ 1,844      
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 16,815      
Carrying Amount at Which Carried [Abstract]        
Land 6,199      
Buildings and Improvements 32,946      
Total 39,145      
Accumulated Depreciation at December 31, 2016 $ 12,591      
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] | Metroplex (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 1,300      
Carrying Amount at Which Carried [Abstract]        
Land 4,373      
Buildings and Improvements 25,846      
Total 30,219      
Accumulated Depreciation at December 31, 2016 $ 6,243      
Real Estate And Accumulated Depreciation Construction Year 2007      
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 3,337      
Carrying Amount at Which Carried [Abstract]        
Land 3,651      
Buildings and Improvements 17,851      
Total 21,502      
Accumulated Depreciation at December 31, 2016 $ 6,986      
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 2,295      
Carrying Amount at Which Carried [Abstract]        
Land 3,652      
Buildings and Improvements 17,583      
Total 21,235      
Accumulated Depreciation at December 31, 2016 $ 6,436      
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 1,630      
Carrying Amount at Which Carried [Abstract]        
Land 3,558      
Buildings and Improvements 16,373      
Total 19,931      
Accumulated Depreciation at December 31, 2016 $ 6,002      
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 1,119      
Carrying Amount at Which Carried [Abstract]        
Land 3,572      
Buildings and Improvements 15,554      
Total 19,126      
Accumulated Depreciation at December 31, 2016 $ 5,778      
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 19,487      
Carrying Amount at Which Carried [Abstract]        
Land 5,405      
Buildings and Improvements 23,263      
Total 28,668      
Accumulated Depreciation at December 31, 2016 $ 3,359      
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] | 351 Plymouth Road [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 1,043      
Buildings and Improvements 555      
Total 1,598      
Accumulated Depreciation at December 31, 2016 $ 163      
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 11,990      
Carrying Amount at Which Carried [Abstract]        
Land 11,897      
Buildings and Improvements 49,004      
Total 60,901      
Accumulated Depreciation at December 31, 2016 $ 20,032      
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 23,052      
Carrying Amount at Which Carried [Abstract]        
Land 7,323      
Buildings and Improvements 51,665      
Total 58,988      
Accumulated Depreciation at December 31, 2016 $ 23,923      
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 4,587      
Carrying Amount at Which Carried [Abstract]        
Land 8,949      
Buildings and Improvements 34,405      
Total 43,354      
Accumulated Depreciation at December 31, 2016 $ 17,840      
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 17,847      
Carrying Amount at Which Carried [Abstract]        
Land 8,609      
Buildings and Improvements 33,760      
Total 42,369      
Accumulated Depreciation at December 31, 2016 $ 15,559      
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 13,271      
Carrying Amount at Which Carried [Abstract]        
Land 5,705      
Buildings and Improvements 34,362      
Total 40,067      
Accumulated Depreciation at December 31, 2016 $ 12,531      
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 5,540      
Carrying Amount at Which Carried [Abstract]        
Land 6,578      
Buildings and Improvements 30,993      
Total 37,571      
Accumulated Depreciation at December 31, 2016 $ 10,392      
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 2,615      
Carrying Amount at Which Carried [Abstract]        
Land 4,791      
Buildings and Improvements 20,558      
Total 25,349      
Accumulated Depreciation at December 31, 2016 $ 9,654      
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 4,075      
Carrying Amount at Which Carried [Abstract]        
Land 3,942      
Buildings and Improvements 19,554      
Total 23,496      
Accumulated Depreciation at December 31, 2016 $ 8,510      
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, 2016 $ 5,433      
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 2,864      
Carrying Amount at Which Carried [Abstract]        
Land 2,509      
Buildings and Improvements 11,016      
Total 13,525      
Accumulated Depreciation at December 31, 2016 $ 3,992      
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      
Buildings and Improvements 0      
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, 2016 $ 373      
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 3,170      
Carrying Amount at Which Carried [Abstract]        
Land 6,251      
Buildings and Improvements 28,379      
Total 34,630      
Accumulated Depreciation at December 31, 2016 $ 8,912      
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 3,125      
Carrying Amount at Which Carried [Abstract]        
Land 3,557      
Buildings and Improvements 17,374      
Total 20,931      
Accumulated Depreciation at December 31, 2016 $ 4,687      
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] | Cira Centre (2929 Arch Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 208,570      
Net Improvements (Retirements) Since Acquisition (9,473)      
Carrying Amount at Which Carried [Abstract]        
Land 12,586      
Buildings and Improvements 186,511      
Total 199,097      
Accumulated Depreciation at December 31, 2016 $ 63,153      
Real Estate And Accumulated Depreciation Construction Year 2005      
Depreciable Life (Years) 40 years      
Philadelphia CBD [Member] | Three Logan Square (1717 Arch Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 98,188      
Net Improvements (Retirements) Since Acquisition 68,691      
Carrying Amount at Which Carried [Abstract]        
Land 25,195      
Buildings and Improvements 141,684      
Total 166,879      
Accumulated Depreciation at December 31, 2016 $ 33,059      
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] | Two Commerce Square (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 21,518      
Carrying Amount at Which Carried [Abstract]        
Land 15,323      
Buildings and Improvements 141,718      
Total 157,041      
Accumulated Depreciation at December 31, 2016 $ 12,190      
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] | One Logan Square (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 27,447      
Carrying Amount at Which Carried [Abstract]        
Land 14,473      
Buildings and Improvements 135,206      
Total 149,679      
Accumulated Depreciation at December 31, 2016 $ 46,840      
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] | Two Logan Square (100 North 18th Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 86,012      
Initial Cost [Abstract]        
Land 16,066      
Buildings and Improvements 100,255      
Net Improvements (Retirements) Since Acquisition 17,631      
Carrying Amount at Which Carried [Abstract]        
Land 16,066      
Buildings and Improvements 117,886      
Total 133,952      
Accumulated Depreciation at December 31, 2016 $ 37,378      
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] | One Commerce Square (2005 Market Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 127,026      
Initial Cost [Abstract]        
Land 15,161      
Buildings and Improvements 105,021      
Net Improvements (Retirements) Since Acquisition 21,419      
Carrying Amount at Which Carried [Abstract]        
Land 15,160      
Buildings and Improvements 126,441      
Total 141,601      
Accumulated Depreciation at December 31, 2016 $ 11,190      
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 $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 76,008      
Net Improvements (Retirements) Since Acquisition 7,588      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 83,596      
Total 83,596      
Accumulated Depreciation at December 31, 2016 $ 12,504      
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 27,176      
Carrying Amount at Which Carried [Abstract]        
Land 7,768      
Buildings and Improvements 44,439      
Total 52,207      
Accumulated Depreciation at December 31, 2016 $ 4,389      
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]        
Land 0      
Buildings and Improvements 21,417      
Net Improvements (Retirements) Since Acquisition 7,651      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 29,068      
Total 29,068      
Accumulated Depreciation at December 31, 2016 $ 6,105      
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] | The Lift at Juniper Street (101 - 103 Juniper Street) [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
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, 2016 $ 2,915      
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 4,317      
Carrying Amount at Which Carried [Abstract]        
Land 628      
Buildings and Improvements 6,417      
Total 7,045      
Accumulated Depreciation at December 31, 2016 $ 2,965      
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      
Net Improvements (Retirements) Since Acquisition 460      
Carrying Amount at Which Carried [Abstract]        
Land 13,365      
Buildings and Improvements 6,251      
Total 19,616      
Accumulated Depreciation at December 31, 2016 $ 2,056      
Real Estate And Accumulated Depreciation Construction Year 1966      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 5 years      
Philadelphia CBD [Member] | FMC Tower at Cira Centre South [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 0      
Buildings and Improvements 166,474      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 166,474      
Total 166,474      
Accumulated Depreciation at December 31, 2016 $ 3,325      
Real Estate And Accumulated Depreciation Construction Year 2016      
Depreciable Life (Years) 40 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 (8,065)      
Carrying Amount at Which Carried [Abstract]        
Land 1,472      
Buildings and Improvements 10,955      
Total 12,427      
Accumulated Depreciation at December 31, 2016 $ 6,066      
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 (10,823)      
Carrying Amount at Which Carried [Abstract]        
Land 257      
Buildings and Improvements 3,809      
Total 4,066      
Accumulated Depreciation at December 31, 2016 $ 2,999      
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 (9,060)      
Carrying Amount at Which Carried [Abstract]        
Land 408      
Buildings and Improvements 3,910      
Total 4,318      
Accumulated Depreciation at December 31, 2016 $ 2,615      
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] | 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 (159)      
Carrying Amount at Which Carried [Abstract]        
Land 155      
Buildings and Improvements 754      
Total 909      
Accumulated Depreciation at December 31, 2016 $ 255      
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]        
Land 0      
Buildings and Improvements 37,421      
Net Improvements (Retirements) Since Acquisition 10,700      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 48,121      
Total 48,121      
Accumulated Depreciation at December 31, 2016 $ 11,894      
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] | 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 5,188      
Carrying Amount at Which Carried [Abstract]        
Land 16,129      
Buildings and Improvements 70,783      
Total 86,912      
Accumulated Depreciation at December 31, 2016 $ 21,568      
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 12,543      
Carrying Amount at Which Carried [Abstract]        
Land 8,782      
Buildings and Improvements 64,417      
Total 73,199      
Accumulated Depreciation at December 31, 2016 $ 17,417      
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] | 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 3,411      
Carrying Amount at Which Carried [Abstract]        
Land 11,472      
Buildings and Improvements 49,304      
Total 60,776      
Accumulated Depreciation at December 31, 2016 $ 12,696      
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 4,510      
Carrying Amount at Which Carried [Abstract]        
Land 10,365      
Buildings and Improvements 48,386      
Total 58,751      
Accumulated Depreciation at December 31, 2016 $ 15,120      
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 18,759      
Carrying Amount at Which Carried [Abstract]        
Land 7,417      
Buildings and Improvements 64,961      
Total 72,378      
Accumulated Depreciation at December 31, 2016 $ 13,783      
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 4,458      
Carrying Amount at Which Carried [Abstract]        
Land 7,373      
Buildings and Improvements 43,534      
Total 50,907      
Accumulated Depreciation at December 31, 2016 $ 10,044      
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 2,793      
Carrying Amount at Which Carried [Abstract]        
Land 5,795      
Buildings and Improvements 41,235      
Total 47,030      
Accumulated Depreciation at December 31, 2016 $ 9,814      
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 6,051      
Carrying Amount at Which Carried [Abstract]        
Land 4,809      
Buildings and Improvements 40,144      
Total 44,953      
Accumulated Depreciation at December 31, 2016 $ 9,918      
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 2,485      
Carrying Amount at Which Carried [Abstract]        
Land 3,866      
Buildings and Improvements 21,778      
Total 25,644      
Accumulated Depreciation at December 31, 2016 $ 6,283      
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 3,307      
Carrying Amount at Which Carried [Abstract]        
Land 18,785      
Buildings and Improvements 100,497      
Total 119,282      
Accumulated Depreciation at December 31, 2016 $ 21,039      
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 2,873      
Carrying Amount at Which Carried [Abstract]        
Land 8,102      
Buildings and Improvements 36,687      
Total 44,789      
Accumulated Depreciation at December 31, 2016 $ 9,000      
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 4,153      
Carrying Amount at Which Carried [Abstract]        
Land 5,237      
Buildings and Improvements 35,193      
Total 40,430      
Accumulated Depreciation at December 31, 2016 $ 8,885      
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 8,227      
Carrying Amount at Which Carried [Abstract]        
Land 5,154      
Buildings and Improvements 37,800      
Total 42,954      
Accumulated Depreciation at December 31, 2016 $ 10,439      
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 18,863      
Carrying Amount at Which Carried [Abstract]        
Land 4,733      
Buildings and Improvements 45,731      
Total 50,464      
Accumulated Depreciation at December 31, 2016 $ 10,051      
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 12,386      
Carrying Amount at Which Carried [Abstract]        
Land 7,944      
Buildings and Improvements 60,056      
Total 68,000      
Accumulated Depreciation at December 31, 2016 $ 15,234      
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 6,201      
Carrying Amount at Which Carried [Abstract]        
Land 4,397      
Buildings and Improvements 37,005      
Total 41,402      
Accumulated Depreciation at December 31, 2016 $ 8,321      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 51 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 144      
Carrying Amount at Which Carried [Abstract]        
Land 3,755      
Buildings and Improvements 22,846      
Total 26,601      
Accumulated Depreciation at December 31, 2016 $ 1,071      
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 1,518      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 17,823      
Total 20,555      
Accumulated Depreciation at December 31, 2016 $ 936      
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 142      
Carrying Amount at Which Carried [Abstract]        
Land 3,688      
Buildings and Improvements 22,490      
Total 26,178      
Accumulated Depreciation at December 31, 2016 $ 1,055      
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 100      
Carrying Amount at Which Carried [Abstract]        
Land 2,614      
Buildings and Improvements 15,840      
Total 18,454      
Accumulated Depreciation at December 31, 2016 $ 743      
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 142      
Carrying Amount at Which Carried [Abstract]        
Land 3,689      
Buildings and Improvements 22,496      
Total 26,185      
Accumulated Depreciation at December 31, 2016 $ 1,055      
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 47      
Carrying Amount at Which Carried [Abstract]        
Land 1,400      
Buildings and Improvements 7,469      
Total 8,869      
Accumulated Depreciation at December 31, 2016 $ 357      
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]        
Land 0      
Buildings and Improvements 19,826      
Net Improvements (Retirements) Since Acquisition 126      
Carrying Amount at Which Carried [Abstract]        
Land 0      
Buildings and Improvements 19,952      
Total 19,952      
Accumulated Depreciation at December 31, 2016 $ 1,248      
Real Estate And Accumulated Depreciation Construction Year 1991      
Real Estate And Accumulated Depreciation Year Of Acquisition 2015      
Depreciable Life (Years) 35 years      
Other [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 (844)      
Carrying Amount at Which Carried [Abstract]        
Land 1,440      
Buildings and Improvements 8,060      
Total 9,500      
Accumulated Depreciation at December 31, 2016 $ 3,995      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
Other [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 (713)      
Carrying Amount at Which Carried [Abstract]        
Land 1,492      
Buildings and Improvements 8,737      
Total 10,229      
Accumulated Depreciation at December 31, 2016 $ 4,093      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
Other [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 564      
Carrying Amount at Which Carried [Abstract]        
Land 1,145      
Buildings and Improvements 7,643      
Total 8,788      
Accumulated Depreciation at December 31, 2016 $ 3,618      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 2001      
Depreciable Life (Years) 40 years      
Other [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 (5,168)      
Carrying Amount at Which Carried [Abstract]        
Land 4,748      
Buildings and Improvements 21,143      
Total 25,891      
Accumulated Depreciation at December 31, 2016 $ 10,481      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 34 years      
Other [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 (5,463)      
Carrying Amount at Which Carried [Abstract]        
Land 4,719      
Buildings and Improvements 21,260      
Total 25,979      
Accumulated Depreciation at December 31, 2016 $ 10,500      
Real Estate And Accumulated Depreciation Construction Year 1984      
Real Estate And Accumulated Depreciation Year Of Acquisition 2006      
Depreciable Life (Years) 34 years      
Other [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 500      
Carrying Amount at Which Carried [Abstract]        
Land 719      
Buildings and Improvements 3,605      
Total 4,324      
Accumulated Depreciation at December 31, 2016 $ 1,815      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 69      
Carrying Amount at Which Carried [Abstract]        
Land 244      
Buildings and Improvements 1,040      
Total 1,284      
Accumulated Depreciation at December 31, 2016 $ 544      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 75      
Carrying Amount at Which Carried [Abstract]        
Land 231      
Buildings and Improvements 996      
Total 1,227      
Accumulated Depreciation at December 31, 2016 $ 520      
Real Estate And Accumulated Depreciation Construction Year 1983      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 58      
Carrying Amount at Which Carried [Abstract]        
Land 185      
Buildings and Improvements 788      
Total 973      
Accumulated Depreciation at December 31, 2016 $ 382      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 30      
Carrying Amount at Which Carried [Abstract]        
Land 183      
Buildings and Improvements 756      
Total 939      
Accumulated Depreciation at December 31, 2016 $ 376      
Real Estate And Accumulated Depreciation Construction Year 1974      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 30      
Carrying Amount at Which Carried [Abstract]        
Land 93      
Buildings and Improvements 394      
Total 487      
Accumulated Depreciation at December 31, 2016 $ 212      
Real Estate And Accumulated Depreciation Construction Year 1972      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 2      
Carrying Amount at Which Carried [Abstract]        
Land 21      
Buildings and Improvements 83      
Total 104      
Accumulated Depreciation at December 31, 2016 $ 43      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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, 2016 $ 32      
Real Estate And Accumulated Depreciation Construction Year 1968      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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, 2016 $ 1,897      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 1,456      
Carrying Amount at Which Carried [Abstract]        
Land 703      
Buildings and Improvements 4,275      
Total 4,978      
Accumulated Depreciation at December 31, 2016 $ 1,996      
Real Estate And Accumulated Depreciation Construction Year 1986      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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, 2016 $ 1,377      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 608      
Carrying Amount at Which Carried [Abstract]        
Land 539      
Buildings and Improvements 2,776      
Total 3,315      
Accumulated Depreciation at December 31, 2016 $ 1,305      
Real Estate And Accumulated Depreciation Construction Year 1987      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [Member] | 7000 Midlantic Derive [Member]        
Real Estate and Accumulated Depreciation [Line Items]        
Encumbrances $ 0      
Initial Cost [Abstract]        
Land 2,560      
Buildings and Improvements 2,790      
Net Improvements (Retirements) Since Acquisition 0      
Carrying Amount at Which Carried [Abstract]        
Land 2,560      
Buildings and Improvements 2,790      
Total 5,350      
Accumulated Depreciation at December 31, 2016 $ 51      
Real Estate And Accumulated Depreciation Construction Year 2016      
Real Estate And Accumulated Depreciation Year Of Acquisition 2003      
Depreciable Life (Years) 40 years      
Other [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 95      
Carrying Amount at Which Carried [Abstract]        
Land 2,732      
Buildings and Improvements 11,037      
Total 13,769      
Accumulated Depreciation at December 31, 2016 $ 11,012      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 3,442      
Carrying Amount at Which Carried [Abstract]        
Land 696      
Buildings and Improvements 6,244      
Total 6,940      
Accumulated Depreciation at December 31, 2016 $ 2,375      
Real Estate And Accumulated Depreciation Construction Year 1990      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 525      
Carrying Amount at Which Carried [Abstract]        
Land 532      
Buildings and Improvements 2,577      
Total 3,109      
Accumulated Depreciation at December 31, 2016 $ 1,379      
Real Estate And Accumulated Depreciation Construction Year 1988      
Real Estate And Accumulated Depreciation Year Of Acquisition 1997      
Depreciable Life (Years) 40 years      
Other [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 3,599      
Carrying Amount at Which Carried [Abstract]        
Land 6,141      
Buildings and Improvements 24,739      
Total 30,880      
Accumulated Depreciation at December 31, 2016 $ 9,090      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 30 years      
Other [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 3,032      
Carrying Amount at Which Carried [Abstract]        
Land 6,369      
Buildings and Improvements 16,771      
Total 23,140      
Accumulated Depreciation at December 31, 2016 $ 8,202      
Real Estate And Accumulated Depreciation Construction Year 1989      
Real Estate And Accumulated Depreciation Year Of Acquisition 2004      
Depreciable Life (Years) 23 years