Segment Information - Income Statement (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Operating revenue		$ 183,130,000	$ 186,654,000	$ 358,496,000	$ 376,795,000
Net operating income		63,271,000	67,533,000	122,066,000	128,776,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(21,942,000)	(30,131,000)	(44,285,000)	(61,543,000)
Depreciation and amortization expense				(92,310,000)	(109,047,000)
Gain on property dispositions		5,895,000	3,832,000	6,702,000	24,353,000
Equity in earnings of unconsolidated joint ventures		3,990,000	5,583,000	9,721,000	10,497,000
General and administrative expense		(15,282,000)	(15,629,000)	(32,224,000)	(36,619,000)
Income taxes		(324,000)	(752,000)	(946,000)	(1,553,000)
Net income		52,818,000	51,057,000	97,062,000	110,120,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		164,551,000	186,547,000	328,631,000	376,960,000
Net operating income		110,518,000	120,544,000	217,720,000	240,176,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		320,000	107,000	121,000	(165,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(21,942,000)	(30,131,000)	(44,285,000)	(61,543,000)
Depreciation and amortization expense	[1]	(33,115,000)	(39,161,000)	(66,308,000)	(78,959,000)
Gain on property dispositions		5,895,000	3,832,000	6,702,000	24,353,000
Equity in earnings of unconsolidated joint ventures		3,990,000	5,583,000	9,721,000	10,497,000
General and administrative expense	[1]	(11,047,000)	(10,135,000)	(23,301,000)	(24,927,000)
Income taxes	[1]	104,000	(653,000)	(176,000)	(1,222,000)
Other		(1,585,000)	1,178,000	(3,011,000)	1,745,000
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		18,259,000	0	29,744,000	0
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		18,225,000	16,174,000	36,282,000	31,934,000
Net operating income		13,094,000	11,219,000	26,077,000	22,059,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,767,000	21,891,000	31,642,000	43,430,000
Net operating income		10,011,000	11,872,000	19,360,000	23,243,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,482,000	27,798,000	28,730,000	57,115,000
Net operating income		10,003,000	16,741,000	19,510,000	34,788,000
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,779,000	14,204,000	29,553,000	28,971,000
Net operating income		7,761,000	8,036,000	14,562,000	16,869,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		39,815,000	33,782,000	81,350,000	68,025,000
Net operating income		29,083,000	24,755,000	58,661,000	48,957,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,928,000	10,386,000	22,363,000	20,741,000
Net operating income		8,475,000	7,981,000	17,234,000	15,339,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,127,000	24,343,000	29,742,000	50,226,000
Net operating income		8,507,000	14,549,000	16,619,000	28,300,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,262,000	3,525,000	6,431,000	7,077,000
Net operating income		1,649,000	2,438,000	3,506,000	4,842,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		32,166,000	34,444,000	62,538,000	69,441,000
Net operating income		$ 21,935,000	$ 22,953,000	$ 42,191,000	$ 45,779,000

[1] Excludes costs which are included in determining SNOI.