Segment Information - Operating Revenues by Segment and Type (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Revenue from External Customer [Line Items]
Operating revenue		$ 199,382	$ 198,972	$ 203,518	$ 206,901	$ 203,685	$ 198,356	$ 192,960	$ 197,630	$ 808,773	$ 792,631	$ 645,930
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										808,683	797,723	758,965
Segment Reconciling Items [Member]
Revenue from External Customer [Line Items]
Operating revenue										0	(4,728)	(113,586)
Corporate, Non-Segment [Member]
Revenue from External Customer [Line Items]
Operating revenue										90	(364)	551
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue										40,917	35,870	30,243
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue										37,431	33,068	18,083
North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										54,031	49,309	37,773
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										133,473	118,145	101,101
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue										47,932	54,892	62,415
North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue										21,802	32,674	31,400
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										40,490	35,238	31,690
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue										31,033	40,911	40,247
North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										50,638	48,804	38,464
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										147,712	148,714	165,248
North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue										54,744	53,725	52,230
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	[1]									133,345	130,013	142,402
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										15,135	16,360	7,669
Industrial Property [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										514,896	477,788	368,921
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue										40,917	35,870	30,243
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue										37,431	33,068	18,083
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										53,802	49,309	37,773
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										131,168	115,821	98,715
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue										24,721	25,378	32,043
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue										20,056	18,635	16,535
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										11,809	10,072	9,534
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue										21,158	20,924	19,073
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										20,740	18,603	7,955
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										28,457	28,895	27,076
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue										26,353	25,451	24,331
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	[1]									86,180	82,672	43,355
Industrial Property [Member] | UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										12,104	13,090	4,205
Office Building [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue										293,787	319,935	390,044
Office Building [Member] | North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue										0	0	0
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] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue										229	0	0
Office Building [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										2,305	2,324	2,386
Office Building [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue										23,211	29,514	30,372
Office Building [Member] | North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue										1,746	14,039	14,865
Office Building [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue										28,681	25,166	22,156
Office Building [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue										9,875	19,987	21,174
Office Building [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue										29,898	30,201	30,509
Office Building [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue										119,255	119,819	138,172
Office Building [Member] | North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue										28,391	28,274	27,899
Office Building [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	[1]									47,165	47,341	99,047
Office Building [Member] | UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue										$ 3,031	$ 3,270	$ 3,464

[1] A substantial portion of the buildings contributing to office operating revenue in 2013 were sold in conjunction with the Portfolio Sale. See Footnote 19.