Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Operating revenue		$ 198,972,000	$ 198,356,000	$ 609,392,000	$ 588,946,000
Net operating income		70,274,000	66,330,000	192,290,000	188,145,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(33,559,000)	(37,958,000)	(103,295,000)	(115,635,000)
Depreciation and amortization expense				(173,726,000)	(176,207,000)
Impairment - real estate assets		0	0	(16,775,000)	0
Gain (loss) on property dispositions		53,467,000	(20,000)	56,987,000	1,878,000
Equity in (loss) earnings of unconsolidated joint ventures		(847,000)	1,592,000	805,000	7,297,000
General and administrative expense		(15,573,000)	(14,748,000)	(51,428,000)	(48,077,000)
Income taxes		(599,000)	(859,000)	(2,613,000)	(2,083,000)
Income from continuing operations		93,695,000	35,318,000	162,085,000	91,388,000
Richmond Hampton Roads [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		198,992,000	198,350,000	609,549,000	593,962,000
Net operating income		125,635,000	124,521,000	382,365,000	366,588,000
Operating Segments [Member] | Carolinas [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,555,000	9,479,000	30,462,000	26,282,000
Net operating income		7,795,000	6,570,000	21,479,000	18,006,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		8,872,000	8,004,000	27,596,000	24,727,000
Net operating income		5,792,000	5,382,000	18,660,000	15,518,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,981,000	12,833,000	38,455,000	36,828,000
Net operating income		7,829,000	7,510,000	22,739,000	21,794,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		33,746,000	30,034,000	100,144,000	87,853,000
Net operating income		24,289,000	21,634,000	70,907,000	60,647,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,751,000	13,925,000	36,699,000	41,217,000
Net operating income		4,461,000	6,575,000	15,595,000	19,964,000
Operating Segments [Member] | Orlando [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		5,547,000	8,463,000	16,221,000	24,519,000
Net operating income		3,835,000	5,411,000	11,091,000	16,257,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,552,000	8,303,000	30,422,000	26,402,000
Net operating income		7,898,000	6,736,000	22,230,000	20,247,000
Operating Segments [Member] | Richmond Hampton Roads [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		5,168,000	10,358,000	25,809,000	30,542,000
Net operating income		3,014,000	6,144,000	15,538,000	18,118,000
Operating Segments [Member] | South Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,957,000	12,012,000	37,542,000	36,028,000
Net operating income		7,012,000	6,647,000	20,990,000	21,066,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		36,718,000	36,087,000	114,430,000	110,833,000
Net operating income		20,887,000	20,516,000	63,301,000	60,175,000
Operating Segments [Member] | Tampa [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		13,568,000	13,597,000	40,667,000	40,001,000
Net operating income		8,454,000	8,213,000	25,780,000	25,142,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,776,000	4,111,000	11,466,000	12,430,000
Net operating income		2,619,000	2,468,000	7,917,000	8,040,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		32,801,000	31,144,000	99,636,000	96,300,000
Net operating income		21,750,000	20,715,000	66,138,000	61,614,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	(9,000)	0	(4,728,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(20,000)	15,000	(157,000)	(288,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(33,559,000)	(37,958,000)	(103,295,000)	(116,192,000)
Depreciation and amortization expense	[1],[2]	(40,870,000)	(44,538,000)	(126,162,000)	(133,636,000)
Impairment - real estate assets		0	0	(16,775,000)	0
Gain (loss) on property dispositions		53,467,000	(20,000)	56,987,000	1,878,000
Equity in (loss) earnings of unconsolidated joint ventures		(847,000)	1,592,000	805,000	7,297,000
General and administrative expense	[1],[2]	(9,456,000)	(8,556,000)	(33,043,000)	(29,189,000)
Discontinued operations excluding interest and gain on property dispositions		0	(95,000)	0	(2,541,000)
Income taxes	[2]	(526,000)	(821,000)	(2,283,000)	(1,966,000)
Other		$ (149,000)	$ 1,193,000	$ 3,486,000	$ (851,000)

[1] Includes activity on discontinued operations.

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