Segment Information - Operating Revenues by Segment and Type (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Revenue from External Customer [Line Items]
Operating revenue	$ 203,685,000	$ 198,356,000	$ 192,960,000	$ 197,630,000	$ 192,427,000	$ 155,349,000	$ 151,026,000	$ 147,128,000	$ 792,631,000		$ 645,930,000		$ 560,279,000
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									797,723,000		758,965,000		697,298,000
Segment Reconciling Items [Member]
Revenue from External Customer [Line Items]
Operating revenue									(4,728,000)		(113,586,000)		(137,476,000)
Corporate, Non-Segment [Member]
Revenue from External Customer [Line Items]
Operating revenue									(364,000)		551,000		457,000
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue									35,870,000		30,243,000		28,667,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									33,068,000		18,083,000		9,780,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									49,309,000		37,773,000		30,172,000
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									118,145,000		101,101,000		96,804,000
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									54,892,000		62,415,000		57,331,000
North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue									32,674,000		31,400,000		30,844,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									35,238,000		31,690,000		28,153,000
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue									40,911,000		40,247,000		41,395,000
North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									48,804,000		38,464,000		35,628,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									148,714,000		165,248,000		169,645,000
North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue									53,725,000		52,230,000		46,627,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									130,013,000	[1]	142,402,000	[1]	117,562,000	[1]
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									16,360,000		7,669,000		4,690,000
Industrial Property [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									477,788,000		368,921,000		309,810,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue									35,870,000		30,243,000		26,818,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									33,068,000		18,083,000		6,908,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									49,309,000		37,773,000		30,172,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									115,821,000		98,715,000		94,440,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									25,378,000		32,043,000		27,810,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue									18,635,000		16,535,000		15,840,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									10,072,000		9,534,000		8,356,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue									20,924,000		19,073,000		18,490,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									18,603,000		7,955,000		5,213,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									28,895,000		27,076,000		28,604,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue									25,451,000		24,331,000		17,584,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									82,672,000	[1]	43,355,000	[1]	28,268,000	[1]
Industrial Property [Member] | UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									13,090,000		4,205,000		1,307,000
Office Building [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									319,935,000		390,044,000		387,488,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue									0		0		1,849,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue									0		0		2,872,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue									0		0		0
Office Building [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									2,324,000		2,386,000		2,364,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue									29,514,000		30,372,000		29,521,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Orlando [Member]
Revenue from External Customer [Line Items]
Operating revenue									14,039,000		14,865,000		15,004,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue									25,166,000		22,156,000		19,797,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue									19,987,000		21,174,000		22,905,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue									30,201,000		30,509,000		30,415,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue									119,819,000		138,172,000		141,041,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Tampa [Member]
Revenue from External Customer [Line Items]
Operating revenue									28,274,000		27,899,000		29,043,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue									47,341,000	[1]	99,047,000	[1]	89,294,000	[1]
Office Building [Member] | UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue									$ 3,270,000		$ 3,464,000		$ 3,383,000

[1] A substantial portion of the buildings contributing to office operating revenue in 2012 and 2013 were sold in conjunction with the Portfolio Sale. See Footnote 19.