Segment Information - Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		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
Segment Reporting Information [Line Items]
Operating revenue		$ 199,382,000	$ 198,972,000	$ 203,518,000	$ 206,901,000	$ 203,685,000	$ 198,356,000	$ 192,960,000	$ 197,630,000	$ 808,773,000	$ 792,631,000	$ 645,930,000
Operating Income (Loss)										257,132,000	257,306,000	202,799,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense										(135,779,000)	(151,887,000)	(127,115,000)
Depreciation and amortization expense										(229,779,000)	(236,075,000)	(205,147,000)
Impairment - real estate assets										(18,244,000)	(117,000)	(248,000)
Gain on property dispositions										100,314,000	45,147,000	0
Equity in earnings of unconsolidated joint ventures										3,149,000	10,314,000	6,067,000
General and administrative expense										(68,710,000)	(63,327,000)	(74,564,000)
Income taxes										(3,233,000)	(2,967,000)	(2,799,000)
Income from continuing operations		$ 82,363,000	$ 93,695,000	$ 36,526,000	$ 31,862,000	$ 84,193,000	$ 35,318,000	$ 29,811,000	$ 26,260,000	244,446,000	175,582,000	97,755,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										808,683,000	797,723,000	758,965,000
Operating Income (Loss)										504,774,000	493,234,000	450,678,000
Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue										0	(4,728,000)	(113,586,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue										90,000	(364,000)	551,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]									(135,779,000)	(152,444,000)	(143,018,000)
Depreciation and amortization expense	[1],[2]									(166,500,000)	(177,715,000)	(138,128,000)
Impairment - real estate assets										(18,244,000)	(117,000)	(248,000)
Gain on property dispositions										100,314,000	45,147,000	0
Equity in earnings of unconsolidated joint ventures										3,149,000	10,314,000	6,067,000
General and administrative expense	[1],[2]									(44,529,000)	(38,041,000)	(50,998,000)
Discontinued operations excluding gain on property dispositions										0	1,950,000	26,455,000
Income taxes	[2]									(2,911,000)	(2,781,000)	(2,748,000)
Other										4,172,000	(65,000)	2,605,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue										0	0	0
North America [Member] | Operating Segments [Member] | Carolinas [Member]
Segment Reporting Information [Line Items]
Operating revenue										40,917,000	35,870,000	30,243,000
Operating Income (Loss)										28,594,000	24,578,000	20,434,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Operating revenue										37,431,000	33,068,000	18,083,000
Operating Income (Loss)										24,851,000	20,856,000	11,726,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Operating revenue										54,031,000	49,309,000	37,773,000
Operating Income (Loss)										31,159,000	29,134,000	22,632,000
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										133,473,000	118,145,000	101,101,000
Operating Income (Loss)										94,972,000	82,050,000	68,504,000
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Operating revenue										47,932,000	54,892,000	62,415,000
Operating Income (Loss)										20,112,000	26,464,000	30,016,000
North America [Member] | Operating Segments [Member] | Orlando [Member]
Segment Reporting Information [Line Items]
Operating revenue										21,802,000	32,674,000	31,400,000
Operating Income (Loss)										14,842,000	21,472,000	20,506,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Operating revenue										40,490,000	35,238,000	31,690,000
Operating Income (Loss)										29,679,000	26,722,000	23,587,000
North America [Member] | Operating Segments [Member] | Richmond Hampton Roads [Member]
Segment Reporting Information [Line Items]
Operating revenue										31,033,000	40,911,000	40,247,000
Operating Income (Loss)										19,255,000	24,357,000	24,063,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										147,712,000	148,714,000	165,248,000
Operating Income (Loss)										80,458,000	81,183,000	91,193,000
North America [Member] | Operating Segments [Member] | South Florida [Member]
Segment Reporting Information [Line Items]
Operating revenue										50,638,000	48,804,000	38,464,000
Operating Income (Loss)										28,308,000	27,947,000	20,943,000
North America [Member] | Operating Segments [Member] | Tampa [Member]
Segment Reporting Information [Line Items]
Operating revenue										54,744,000	53,725,000	52,230,000
Operating Income (Loss)										34,133,000	33,776,000	33,042,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue	[3]									133,345,000	130,013,000	142,402,000
Operating Income (Loss)										87,925,000	83,991,000	81,435,000
UNITED KINGDOM | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue										15,135,000	16,360,000	7,669,000
Operating Income (Loss)										$ 10,486,000	$ 10,704,000	$ 2,597,000

[1] Includes activity on discontinued operations.

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

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