Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Operating revenue		$ 175,366,000	$ 190,140,000
Development service fee income		11,485,000	0
Net operating income		58,795,000	61,243,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(22,343,000)	(31,412,000)
Depreciation and amortization expense		(45,992,000)	(54,837,000)
Gain on property dispositions		807,000	20,521,000
Equity in earnings of unconsolidated joint ventures		5,731,000	4,914,000
General and administrative expense		(16,942,000)	(20,990,000)
Income taxes		(622,000)	(801,000)
Net income		44,244,000	59,063,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		164,079,000	190,414,000
Net operating income		107,207,000	119,371,000
Operating Segments [Member] | Carolinas Richmond [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		18,056,000	15,760,000
Net operating income		12,982,000	10,840,000
Operating Segments [Member] | Chicago Minneapolis [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,875,000	21,539,000
Net operating income		9,349,000	11,371,000
Operating Segments [Member] | Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,248,000	29,318,000
Net operating income		9,511,000	18,047,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,774,000	14,766,000
Net operating income		6,801,000	8,833,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		41,535,000	34,243,000
Net operating income		29,578,000	24,202,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,436,000	10,355,000
Net operating income		8,759,000	7,097,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,615,000	25,883,000
Net operating income		8,113,000	13,751,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,168,000	3,552,000
Net operating income		1,858,000	2,405,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		30,372,000	34,998,000
Net operating income		20,256,000	22,825,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(198,000)	(274,000)
Development service fee income		11,485,000	0
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(22,343,000)	(31,412,000)
Depreciation and amortization expense	[1]	(33,193,000)	(39,798,000)
Gain on property dispositions		807,000	20,521,000
Equity in earnings of unconsolidated joint ventures		5,731,000	4,914,000
General and administrative expense	[1]	(12,254,000)	(14,539,000)
Income taxes	[1]	(280,000)	(570,000)
Other		$ (1,431,000)	$ 576,000

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