Segment Information (Operating Revenues by Segment and Type) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Revenue from External Customer [Line Items]
Operating revenue	$ 179,057	$ 190,857	$ 186,654	$ 190,140	$ 199,382	$ 198,972	$ 203,518	$ 206,901	$ 746,708	$ 808,773	$ 792,631
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									734,050	808,683	797,723
Segment Reconciling Items [Member] | Development Service Fee Income [Member]
Revenue from External Customer [Line Items]
Operating revenue									12,941	0	0
Segment Reconciling Items [Member] | Discontinued Operations [Member]
Revenue from External Customer [Line Items]
Operating revenue									0	0	(4,728)
Corporate, Non-Segment [Member]
Revenue from External Customer [Line Items]
Operating revenue									(283)	90	(364)
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue									66,270	71,950	76,781
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									42,836	37,431	33,068
North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									99,733	127,184	135,203
North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									59,582	54,031	49,309
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									141,046	133,473	118,145
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									41,192	47,932	54,892
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									41,838	40,490	35,238
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									91,656	147,712	148,714
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									136,521	133,345	130,013
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									13,376	15,135	16,360
Industrial Property [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									529,268	514,896	477,788
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue									66,270	62,075	56,794
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									42,836	37,431	33,068
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									66,028	67,149	62,689
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									58,370	53,802	49,309
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									138,766	131,168	115,821
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									22,171	24,721	25,378
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									12,842	11,809	10,072
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									19,448	28,457	28,895
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									91,762	86,180	82,672
Industrial Property [Member] | United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									10,775	12,104	13,090
Office Building [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									204,782	293,787	319,935
Office Building [Member] | North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue									0	9,875	19,987
Office Building [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									0	0	0
Office Building [Member] | North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									33,705	60,035	72,514
Office Building [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									1,212	229	0
Office Building [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									2,280	2,305	2,324
Office Building [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									19,021	23,211	29,514
Office Building [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									28,996	28,681	25,166
Office Building [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									72,208	119,255	119,819
Office Building [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									44,759	47,165	47,341
Office Building [Member] | United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									$ 2,601	$ 3,031	$ 3,270