Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015
Segment Reporting Information [Line Items]
Operating revenue		$ 190,140,000	$ 206,901,000
Net operating income		61,243,000	51,829,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(31,412,000)	(34,670,000)
Depreciation and amortization expense		(54,837,000)	(59,431,000)
Impairment - real estate assets		0	(15,739,000)
Gain on property dispositions		20,521,000	2,271,000
Equity in earnings of unconsolidated joint ventures		4,914,000	6,906,000
General and administrative expense		(20,990,000)	(18,802,000)
Income taxes		(801,000)	(845,000)
Net income		59,063,000	31,862,000
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
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		190,414,000	206,952,000
Net operating income		119,371,000	127,570,000
Operating Segments [Member] | Carolinas Richmond [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,760,000	20,334,000
Net operating income		10,840,000	12,846,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,193,000	9,793,000
Net operating income		6,793,000	6,743,000
Operating Segments [Member] | Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		29,318,000	30,932,000
Net operating income		18,047,000	19,102,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,766,000	12,324,000
Net operating income		8,833,000	7,221,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		34,243,000	33,554,000
Net operating income		24,202,000	22,971,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,346,000	12,672,000
Net operating income		4,578,000	5,985,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,355,000	9,982,000
Net operating income		7,097,000	7,022,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		25,883,000	39,238,000
Net operating income		13,751,000	20,988,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,552,000	3,725,000
Net operating income		2,405,000	2,516,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		34,998,000	34,398,000
Net operating income		22,825,000	22,176,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(274,000)	(51,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(31,412,000)	(34,670,000)
Depreciation and amortization expense	[1]	(39,798,000)	(43,514,000)
Impairment - real estate assets		0	(15,739,000)
Gain on property dispositions		20,521,000	2,271,000
Equity in earnings of unconsolidated joint ventures		4,914,000	6,906,000
General and administrative expense	[1]	(14,539,000)	(12,574,000)
Income taxes	[1]	(570,000)	(707,000)
Other		$ 576,000	$ 2,319,000

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