Performance of the Reportable Segments Based on Property Level Operating Income (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Operating revenue		$ 190,857,000	$ 198,972,000	$ 567,653,000	$ 609,392,000
Net operating income		75,322,000	70,274,000	204,098,000	192,290,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(29,528,000)	(33,559,000)	(91,071,000)	(103,295,000)
Loss on debt extinguishment		(3,494,000)	0	(3,494,000)	0
Depreciation and amortization expense				(156,562,000)	(173,726,000)
Impairment - real estate assets		0	0	0	(16,775,000)
Gain on property dispositions		1,318,000	53,467,000	25,671,000	56,987,000
Equity in earnings (loss) of unconsolidated joint ventures		9,043,000	(847,000)	19,540,000	805,000
General and administrative expense		(16,151,000)	(15,573,000)	(52,770,000)	(51,428,000)
Income taxes		(80,000)	(599,000)	(1,633,000)	(2,613,000)
Net income		55,734,000	93,695,000	165,854,000	162,085,000
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	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		191,033,000	198,992,000	567,994,000	609,549,000
Net operating income		124,671,000	125,635,000	364,845,000	382,365,000
Operating Segments [Member] | Carolinas Richmond [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		17,292,000	15,723,000	49,226,000	56,271,000
Net operating income		12,198,000	10,809,000	34,257,000	37,017,000
Operating Segments [Member] | Chicago Milwaukee [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,384,000	8,872,000	31,896,000	27,596,000
Net operating income		7,103,000	5,792,000	20,917,000	18,660,000
Operating Segments [Member] | Florida [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		28,115,000	32,072,000	85,230,000	94,430,000
Net operating income		18,340,000	19,301,000	53,129,000	57,861,000
Operating Segments [Member] | Houston [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		14,870,000	12,981,000	43,840,000	38,455,000
Net operating income		7,989,000	7,829,000	24,858,000	22,739,000
Operating Segments [Member] | Lehigh Central PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		34,157,000	33,746,000	102,182,000	100,144,000
Net operating income		24,982,000	24,289,000	73,939,000	70,907,000
Operating Segments [Member] | Minnesota [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		12,033,000	11,751,000	34,952,000	36,699,000
Net operating income		5,416,000	4,461,000	14,844,000	15,595,000
Operating Segments [Member] | Philadelphia [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,513,000	10,552,000	31,253,000	30,422,000
Net operating income		7,725,000	7,898,000	23,064,000	22,230,000
Operating Segments [Member] | Southeastern PA [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		24,563,000	36,718,000	74,789,000	114,430,000
Net operating income		16,244,000	20,887,000	44,544,000	63,301,000
Operating Segments [Member] | United Kingdom [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,275,000	3,776,000	10,352,000	11,466,000
Net operating income		2,031,000	2,619,000	6,871,000	7,917,000
Operating Segments [Member] | Other Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		34,831,000	32,801,000	104,274,000	99,636,000
Net operating income		22,643,000	21,750,000	68,422,000	66,138,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(176,000)	(20,000)	(341,000)	(157,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(29,528,000)	(33,559,000)	(91,071,000)	(103,295,000)
Loss on debt extinguishment		(3,494,000)	0	(3,494,000)	0
Depreciation and amortization expense	[1]	(34,896,000)	(40,870,000)	(113,855,000)	(126,162,000)
Impairment - real estate assets		0	0	0	(16,775,000)
Gain on property dispositions		1,318,000	53,467,000	25,671,000	56,987,000
Equity in earnings (loss) of unconsolidated joint ventures		9,043,000	(847,000)	19,540,000	805,000
General and administrative expense	[1]	(10,595,000)	(9,456,000)	(34,209,000)	(33,043,000)
Income taxes	[1]	17,000	(526,000)	(1,203,000)	(2,283,000)
Other		$ (802,000)	$ (149,000)	$ (370,000)	$ 3,486,000

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