Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (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
Segment Reporting Information [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 Income (Loss)										257,423,000		203,047,000		189,942,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense										(151,887,000)		(127,115,000)		(103,363,000)
Depreciation and amortization expense										(236,075,000)		(205,147,000)		(167,421,000)
Gain on property dispositions										45,030,000		0		0
Equity in earnings (loss) of unconsolidated joint ventures										10,314,000		6,067,000		(681,000)
General and Administrative Expense										(63,327,000)		(74,564,000)		(64,686,000)
Income taxes										(2,967,000)		(2,799,000)		(976,000)
Income from continuing operations	84,193,000	[1]	35,318,000	29,811,000	26,260,000	19,911,000	23,406,000	29,611,000	24,827,000	175,582,000		97,755,000		96,747,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										797,723,000		758,965,000		697,298,000
Operating Income (Loss)										493,234,000		450,678,000		409,230,000
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue										(4,728,000)		(113,586,000)		(137,476,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue										(364,000)		551,000		457,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense										(152,444,000)	[2]	(143,018,000)	[2]	(123,146,000)	[2]
Depreciation and amortization expense										(177,715,000)	[3]	(138,128,000)	[3]	(104,643,000)	[3]
Gain on property dispositions										45,030,000		0		0
Equity in earnings (loss) of unconsolidated joint ventures										10,314,000		6,067,000		(681,000)
General and Administrative Expense										(38,041,000)	[3]	(50,998,000)	[3]	(40,831,000)	[3]
Discontinued operations excluding gain on property dispositions										1,950,000		26,455,000		38,578,000
Income taxes										(2,781,000)	[3]	(2,748,000)	[3]	(874,000)	[3]
Other										(65,000)		2,357,000		(3,730,000)
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										118,145,000		101,101,000		96,804,000
Operating Income (Loss)										82,050,000		68,504,000		65,566,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Operating revenue										33,068,000		18,083,000		9,780,000
Operating Income (Loss)										20,856,000		11,726,000		5,329,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Operating revenue										49,309,000		37,773,000		30,172,000
Operating Income (Loss)										29,134,000		22,632,000		17,862,000
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Operating revenue										35,870,000		30,243,000		28,667,000
Operating Income (Loss)										24,578,000		20,434,000		18,733,000
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Operating revenue										54,892,000		62,415,000		57,331,000
Operating Income (Loss)										26,464,000		30,016,000		26,348,000
North America [Member] | Operating Segments [Member] | Orlando [Member]
Segment Reporting Information [Line Items]
Operating revenue										32,674,000		31,400,000		30,844,000
Operating Income (Loss)										21,472,000		20,506,000		20,153,000
North America [Member] | Operating Segments [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Operating revenue										48,804,000		38,464,000		35,628,000
Operating Income (Loss)										27,947,000		20,943,000		18,921,000
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Operating revenue										40,911,000		40,247,000		41,395,000
Operating Income (Loss)										24,357,000		24,063,000		24,762,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										130,013,000	[4]	142,402,000	[4]	117,562,000	[4]
Operating Income (Loss)										83,991,000		81,435,000		64,207,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Operating revenue										35,238,000		31,690,000		28,153,000
Operating Income (Loss)										26,722,000		23,587,000		20,527,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										148,714,000		165,248,000		169,645,000
Operating Income (Loss)										81,183,000		91,193,000		98,729,000
North America [Member] | Operating Segments [Member] | Tampa [Member]
Segment Reporting Information [Line Items]
Operating revenue										53,725,000		52,230,000		46,627,000
Operating Income (Loss)										33,776,000		33,042,000		28,350,000
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue										16,360,000		7,669,000		4,690,000
Operating Income (Loss)										$ 10,704,000		$ 2,597,000		$ (257,000)

[1] Includes gain on sale of $43.2 million.

[2] Includes activity on discontinued operations.

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

[4] 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.