Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Operating revenue		$ 186,654,000	$ 203,518,000	$ 376,795,000	$ 410,419,000
Net operating income		67,533,000	70,185,000	128,776,000	122,014,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(30,131,000)	(35,066,000)	(61,543,000)	(69,736,000)
Depreciation and amortization expense				(109,047,000)	(117,160,000)
Impairment - real estate assets		0	(1,036,000)	0	(16,775,000)
Gain on property dispositions		3,832,000	1,249,000	24,353,000	3,520,000
Equity in earnings (loss) of unconsolidated joint ventures		5,583,000	(5,254,000)	10,497,000	1,652,000
General and administrative expense		(15,629,000)	(17,053,000)	(36,619,000)	(35,855,000)
Income taxes		(752,000)	(1,169,000)	(1,553,000)	(2,014,000)
Net income		51,057,000	36,526,000	110,120,000	68,388,000
Carolinas Richmond [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Impairment - real estate assets					(13,400,000)
Southeastern PA [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Impairment - real estate assets					(2,300,000)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		186,547,000	203,604,000	376,960,000	410,556,000
Net operating income		120,544,000	128,464,000	240,176,000	256,730,000
Operating Segments [Member] | Carolinas Richmond [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		16,174,000	20,212,000	31,934,000	40,547,000
Net operating income		11,219,000	13,361,000	22,059,000	26,208,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,319,000	8,931,000	20,512,000	18,724,000
Net operating income		7,022,000	6,125,000	13,815,000	12,868,000
Operating Segments [Member] | Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		27,798,000	31,424,000	57,115,000	62,358,000
Net operating income		16,741,000	19,459,000	34,788,000	38,561,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,204,000	13,151,000	28,971,000	25,474,000
Net operating income		8,036,000	7,689,000	16,869,000	14,910,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		33,782,000	32,844,000	68,025,000	66,398,000
Net operating income		24,755,000	23,647,000	48,957,000	46,618,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,572,000	12,276,000	22,918,000	24,948,000
Net operating income		4,850,000	5,149,000	9,428,000	11,134,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,386,000	9,889,000	20,741,000	19,871,000
Net operating income		7,981,000	7,310,000	15,339,000	14,332,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		24,343,000	38,473,000	50,226,000	77,712,000
Net operating income		14,549,000	21,426,000	28,300,000	42,414,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,525,000	3,965,000	7,077,000	7,690,000
Net operating income		2,438,000	2,782,000	4,842,000	5,298,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		34,444,000	32,439,000	69,441,000	66,834,000
Net operating income		22,953,000	21,516,000	45,779,000	44,387,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		107,000	(86,000)	(165,000)	(137,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(30,131,000)	(35,066,000)	(61,543,000)	(69,736,000)
Depreciation and amortization expense	[1]	(39,161,000)	(41,772,000)	(78,959,000)	(85,281,000)
Impairment - real estate assets		0	(1,036,000)	0	(16,775,000)
Gain on property dispositions		3,832,000	1,249,000	24,353,000	3,520,000
Equity in earnings (loss) of unconsolidated joint ventures		5,583,000	(5,254,000)	10,497,000	1,652,000
General and administrative expense	[1]	(10,135,000)	(9,128,000)	(24,927,000)	(19,437,000)
Income taxes	[1]	(653,000)	(1,051,000)	(1,222,000)	(1,757,000)
Other		$ 1,178,000	$ 120,000	$ 1,745,000	$ (528,000)

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