Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information [Line Items]
Operating revenue									$ 645,930,000		$ 560,279,000		$ 533,699,000
Operating Income (Loss)									203,047,000		189,942,000		190,995,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(127,115,000)		(103,363,000)		(101,988,000)
Depreciation and amortization expense									(205,147,000)		(167,421,000)		(171,714,000)
Gain on property dispositions									8,676,000		3,080,000		5,025,000
Equity in earnings (loss) of unconsolidated joint ventures									6,067,000		(681,000)		3,496,000
General and Administrative Expense									(74,564,000)		(64,686,000)		(59,186,000)
Income taxes									(2,799,000)		(976,000)		(1,020,000)
Income from continuing operations	19,911,000	23,406,000	29,611,000	24,827,000	23,776,000	22,358,000	24,725,000	25,888,000	97,755,000		96,747,000		104,645,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue									758,965,000		697,298,000		725,214,000
Operating Income (Loss)									450,005,000		409,230,000		421,794,000
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue									(113,586,000)		(137,476,000)		(191,723,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue									551,000		457,000		208,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(143,018,000)	[1]	(123,146,000)	[1]	(131,046,000)	[1]
Depreciation and amortization expense									(138,343,000)	[2]	(104,643,000)	[2]	(106,487,000)	[2]
Gain on property dispositions									8,676,000		3,080,000		5,025,000
Equity in earnings (loss) of unconsolidated joint ventures									6,067,000		(681,000)		3,496,000
General and Administrative Expense									(50,998,000)	[2]	(40,831,000)	[2]	(36,140,000)	[2]
Discontinued operations excluding gain on property dispositions									26,455,000		38,578,000		45,483,000
Income taxes									(2,748,000)	[2]	(874,000)	[2]	(841,000)	[2]
Other									(5,431,000)		(6,810,000)		(5,673,000)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue									0		0		0
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue									101,101,000		96,804,000		99,311,000
Operating Income (Loss)									68,504,000		65,566,000		64,786,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Operating revenue									18,083,000		9,780,000		21,973,000
Operating Income (Loss)									11,726,000		5,329,000		10,581,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Operating revenue									37,773,000		30,172,000		27,325,000
Operating Income (Loss)									22,632,000		17,862,000		16,379,000
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Operating revenue									30,243,000		28,667,000		32,470,000
Operating Income (Loss)									20,434,000		18,733,000		19,471,000
North America [Member] | Operating Segments [Member] | Industrial Other [Member]
Segment Reporting Information [Line Items]
Operating revenue									104,387,000	[3]	93,667,000	[3]	101,004,000	[3]
Operating Income (Loss)									60,531,000		51,863,000		54,802,000
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Operating revenue									62,415,000		57,331,000		55,886,000
Operating Income (Loss)									30,016,000		26,348,000		27,701,000
North America [Member] | Operating Segments [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Operating revenue									38,464,000		35,628,000		36,792,000
Operating Income (Loss)									20,943,000		18,921,000		19,431,000
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Operating revenue									40,247,000		41,395,000		49,714,000
Operating Income (Loss)									24,063,000		24,762,000		29,324,000
North America [Member] | Operating Segments [Member] | Arizona [Member]
Segment Reporting Information [Line Items]
Operating revenue									26,559,000		23,033,000		21,083,000
Operating Income (Loss)									17,189,000		14,228,000		13,453,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue									81,487,000		73,126,000		72,608,000
Operating Income (Loss)									50,456,000		43,711,000		44,692,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Operating revenue									31,690,000		28,153,000		28,129,000
Operating Income (Loss)									23,587,000		20,527,000		20,504,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue									165,248,000		169,645,000		174,063,000
Operating Income (Loss)									91,193,000		98,729,000		101,982,000
North America [Member] | Operating Segments [Member] | Washington DC [Member]
Segment Reporting Information [Line Items]
Operating revenue									13,599,000		5,207,000		448,000
Operating Income (Loss)									6,807,000		2,908,000		(1,134,000)
European Union [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue									7,669,000		4,690,000		4,408,000
Operating Income (Loss)									$ 1,924,000		$ (257,000)		$ (178,000)

[1] Includes interest on discontinued operations.

[2] Excludes costs which are included in determining segment-level net operating income.

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