Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Operating revenue		$ 203,518,000	$ 192,960,000	$ 410,419,000	$ 390,590,000
Net operating income		70,185,000	62,417,000	122,014,000	121,815,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(35,066,000)	(38,470,000)	(69,736,000)	(77,677,000)
Depreciation and amortization expense				(117,160,000)	(116,547,000)
Impairment - real estate assets		(1,036,000)	0	(16,775,000)	0
Gain on property dispositions		1,249,000	1,897,000	3,520,000	1,898,000
Equity in (loss) earnings of unconsolidated joint ventures		(5,254,000)	1,546,000	1,652,000	5,705,000
General and administrative expense		(17,053,000)	(14,973,000)	(35,855,000)	(33,329,000)
Income taxes		(1,169,000)	(693,000)	(2,014,000)	(1,224,000)
Income from continuing operations		36,526,000	29,812,000	68,388,000	56,070,000
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		203,604,000	192,955,000	410,556,000	395,615,000
Net operating income		128,464,000	119,967,000	256,730,000	242,065,000
Operating Segments [Member] | Carolinas [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,062,000	8,486,000	19,906,000	16,803,000
Net operating income		7,066,000	5,693,000	13,684,000	11,435,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		8,931,000	8,152,000	18,724,000	16,723,000
Net operating income		6,125,000	4,973,000	12,868,000	10,136,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		13,151,000	12,128,000	25,474,000	23,995,000
Net operating income		7,689,000	7,232,000	14,910,000	14,284,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		32,844,000	28,195,000	66,398,000	57,819,000
Net operating income		23,647,000	19,773,000	46,618,000	39,013,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,276,000	13,541,000	24,948,000	27,292,000
Net operating income		5,149,000	6,770,000	11,134,000	13,390,000
Operating Segments [Member] | Orlando [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		5,409,000	8,061,000	10,674,000	16,056,000
Net operating income		3,685,000	5,354,000	7,256,000	10,846,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		9,889,000	8,691,000	19,871,000	18,099,000
Net operating income		7,310,000	6,790,000	14,332,000	13,511,000
Operating Segments [Member] | Richmond Hampton Roads [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,150,000	10,115,000	20,641,000	20,184,000
Net operating income		6,295,000	6,054,000	12,524,000	11,973,000
Operating Segments [Member] | South Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,378,000	11,707,000	24,585,000	24,016,000
Net operating income		7,149,000	6,891,000	13,978,000	14,418,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		38,473,000	36,309,000	77,712,000	74,746,000
Net operating income		21,426,000	20,060,000	42,414,000	39,660,000
Operating Segments [Member] | Tampa [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		13,637,000	13,230,000	27,099,000	26,404,000
Net operating income		8,625,000	8,488,000	17,327,000	16,929,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,965,000	4,218,000	7,690,000	8,320,000
Net operating income		2,782,000	2,722,000	5,298,000	5,572,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		32,439,000	30,122,000	66,834,000	65,158,000
Net operating income		21,516,000	19,167,000	44,387,000	40,898,000
Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	29,000	0	(4,719,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(86,000)	(24,000)	(137,000)	(306,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(35,066,000)	(38,470,000)	(69,736,000)	(78,234,000)
Depreciation and amortization expense	[1],[2]	(41,772,000)	(44,561,000)	(85,281,000)	(89,098,000)
Impairment - real estate assets		(1,036,000)	0	(16,775,000)	0
Gain on property dispositions		1,249,000	1,897,000	3,520,000	1,898,000
Equity in (loss) earnings of unconsolidated joint ventures		(5,254,000)	1,546,000	1,652,000	5,705,000
General and administrative expense	[1],[2]	(9,128,000)	(8,232,000)	(19,437,000)	(20,633,000)
Discontinued operations excluding interest and gain on property dispositions		0	(156,000)	0	(2,446,000)
Income taxes	[2]	(1,051,000)	(663,000)	(1,757,000)	(1,146,000)
Other		$ 120,000	$ (1,516,000)	$ (528,000)	$ (2,041,000)

[1] Includes activity on discontinued operations.

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