Segment Information - Operating Revenues by Segment and Type (Details) (USD $)	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Revenue from External Customer [Line Items]
Operating revenue	$ 645,930,000		$ 560,279,000		$ 533,699,000
Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	758,965,000		697,298,000		725,214,000
Segment Reconciling Items [Member]
Revenue from External Customer [Line Items]
Operating revenue	(113,586,000)		(137,476,000)		(191,723,000)
Corporate, Non-Segment [Member]
Revenue from External Customer [Line Items]
Operating revenue	551,000		457,000		208,000
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	101,101,000		96,804,000		99,311,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue	18,083,000		9,780,000		21,973,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue	37,773,000		30,172,000		27,325,000
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue	30,243,000		28,667,000		32,470,000
North America [Member] | Operating Segments [Member] | Industrial Other [Member]
Revenue from External Customer [Line Items]
Operating revenue	104,387,000	[1]	93,667,000	[1]	101,004,000	[1]
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue	62,415,000		57,331,000		55,886,000
North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue	38,464,000		35,628,000		36,792,000
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue	40,247,000		41,395,000		49,714,000
North America [Member] | Operating Segments [Member] | Arizona [Member]
Revenue from External Customer [Line Items]
Operating revenue	26,559,000		23,033,000		21,083,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	81,487,000		73,126,000		72,608,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue	31,690,000		28,153,000		28,129,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	165,248,000		169,645,000		174,063,000
North America [Member] | Operating Segments [Member] | Washington DC [Member]
Revenue from External Customer [Line Items]
Operating revenue	13,599,000		5,207,000		448,000
European Union [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue	7,669,000		4,690,000		4,408,000
Industrial Property [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	368,921,000		309,810,000		302,765,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	98,715,000		94,440,000		93,193,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue	18,083,000		6,908,000		8,239,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Houston [Member]
Revenue from External Customer [Line Items]
Operating revenue	37,773,000		30,172,000		27,325,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue	30,243,000		26,818,000		23,381,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Industrial Other [Member]
Revenue from External Customer [Line Items]
Operating revenue	44,672,000	[1]	32,313,000	[1]	33,847,000	[1]
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue	32,043,000		27,810,000		24,398,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue	7,955,000		5,213,000		5,887,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue	19,073,000		18,490,000		18,889,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Arizona [Member]
Revenue from External Customer [Line Items]
Operating revenue	4,231,000		2,393,000		1,407,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	35,318,000		26,986,000		27,805,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue	9,534,000		8,356,000		7,817,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	27,076,000		28,604,000		29,289,000
Industrial Property [Member] | North America [Member] | Operating Segments [Member] | Washington DC [Member]
Revenue from External Customer [Line Items]
Operating revenue	0		0		0
Industrial Property [Member] | European Union [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue	4,205,000		1,307,000		1,288,000
Office Building [Member] | Operating Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	390,044,000		387,488,000		422,449,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	2,386,000		2,364,000		6,118,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Revenue from External Customer [Line Items]
Operating revenue	0		2,872,000		13,734,000
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] | Carolinas [Member]
Revenue from External Customer [Line Items]
Operating revenue	0		1,849,000		9,089,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Industrial Other [Member]
Revenue from External Customer [Line Items]
Operating revenue	59,715,000	[1]	61,354,000	[1]	67,157,000	[1]
Office Building [Member] | North America [Member] | Operating Segments [Member] | Minnesota [Member]
Revenue from External Customer [Line Items]
Operating revenue	30,372,000		29,521,000		31,488,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | South Florida [Member]
Revenue from External Customer [Line Items]
Operating revenue	30,509,000		30,415,000		30,905,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Revenue from External Customer [Line Items]
Operating revenue	21,174,000		22,905,000		30,825,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Arizona [Member]
Revenue from External Customer [Line Items]
Operating revenue	22,328,000		20,640,000		19,676,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Other Segments [Member]
Revenue from External Customer [Line Items]
Operating revenue	46,169,000		46,140,000		44,803,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Revenue from External Customer [Line Items]
Operating revenue	22,156,000		19,797,000		20,312,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Revenue from External Customer [Line Items]
Operating revenue	138,172,000		141,041,000		144,774,000
Office Building [Member] | North America [Member] | Operating Segments [Member] | Washington DC [Member]
Revenue from External Customer [Line Items]
Operating revenue	13,599,000		5,207,000		448,000
Office Building [Member] | European Union [Member] | Operating Segments [Member] | United Kingdom [Member]
Revenue from External Customer [Line Items]
Operating revenue	$ 3,464,000		$ 3,383,000		$ 3,120,000

[1] A substantial portion of the office revenues were sold in conjunction with the Portfolio Sale. See Footnote 23.