Segment Information (Operating Revenues by Segment and Type) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue from External Customer [Line Items]
Operating revenue		$ 192,803	$ 184,511	$ 174,888	$ 167,576	$ 169,733	$ 181,967	$ 178,033	$ 180,965	$ 719,778	$ 710,698	$ 773,960
Development service fee income	[1]									82,673	12,941	0
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										671,280	734,050	808,683
Segment Reconciling Items [Member] | Discontinued Operations [Member]
Revenue from External Customer [Line Items]
Operating revenue										(34,035)	(36,010)	(34,813)
Corporate, Non-Segment [Member]
Revenue from External Customer [Line Items]
Operating revenue										(140)	(283)	90
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue										74,158	66,270	71,950
North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Revenue from External Customer [Line Items]
Operating revenue										64,706	84,028	85,363
North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										59,526	99,733	127,184
North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										59,957	59,582	54,031
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										163,026	141,046	133,473
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										45,504	41,838	40,490
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										59,762	91,656	147,712
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										130,724	136,521	133,345
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										13,917	13,376	15,135
Industrial Property [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										547,204	529,268	514,896
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue										74,158	66,270	62,075
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Revenue from External Customer [Line Items]
Operating revenue										60,826	65,007	62,152
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										56,923	66,028	67,149
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										58,606	58,370	53,802
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										163,026	138,766	131,168
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										11,914	12,842	11,809
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										8,925	19,448	28,457
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										102,457	91,762	86,180
Industrial Property [Member] | United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										10,369	10,775	12,104
Office Building [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										124,076	204,782	293,787
Office Building [Member] | North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Revenue from External Customer [Line Items]
Operating revenue										0	0	9,875
Office Building [Member] | North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Revenue from External Customer [Line Items]
Operating revenue										3,880	19,021	23,211
Office Building [Member] | North America [Member] | Operating Segments [Member] | Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										2,603	33,705	60,035
Office Building [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										1,351	1,212	229
Office Building [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										0	2,280	2,305
Office Building [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										33,590	28,996	28,681
Office Building [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										50,837	72,208	119,255
Office Building [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										28,267	44,759	47,165
Office Building [Member] | United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										$ 3,548	$ 2,601	$ 3,031

[1] (1) Prior to the fourth quarter of 2016, development service fee income had been classified as other income and development service fee expense had been classified as general and administrative expense. See Note 2.