Segment Information - Income Statement (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Operating revenue		$ 193,653,000	$ 190,857,000	$ 552,149,000	$ 567,653,000
Net operating income		54,875,000	75,322,000	176,941,000	204,098,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(23,060,000)	(29,528,000)	(67,345,000)	(91,071,000)
Loss on debt extinguishment		0	(3,494,000)	0	(3,494,000)
Depreciation and amortization expense		(46,582,000)	(46,920,000)	(137,831,000)	(154,543,000)
Impairment charges - real estate assets		(9,650,000)	0	(9,650,000)	0
Gain on property dispositions		23,840,000	1,318,000	30,542,000	25,671,000
Equity in earnings of unconsolidated joint ventures		4,305,000	9,043,000	14,026,000	19,540,000
General and administrative expense		(11,910,000)	(15,379,000)	(43,949,000)	(51,888,000)
Expensed pursuit costs		(4,772,000)	(772,000)	(4,957,000)	(882,000)
Income taxes		(582,000)	(80,000)	(1,528,000)	(1,633,000)
Net income		61,159,000	55,734,000	158,221,000	165,854,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		169,614,000	191,033,000	498,247,000	567,994,000
Net operating income		113,718,000	125,254,000	331,446,000	365,428,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(137,000)	(176,000)	(18,000)	(341,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(23,060,000)	(29,528,000)	(67,345,000)	(91,071,000)
Loss on debt extinguishment		0	(3,494,000)	0	(3,494,000)
Depreciation and amortization expense	[1]	(33,521,000)	(34,896,000)	(99,829,000)	(113,855,000)
Impairment charges - real estate assets		(9,650,000)	0	(9,650,000)	0
Gain on property dispositions		23,840,000	1,318,000	30,542,000	25,671,000
Equity in earnings of unconsolidated joint ventures		4,305,000	9,043,000	14,026,000	19,540,000
General and administrative expense	[1]	(8,104,000)	(10,406,000)	(31,230,000)	(33,910,000)
Expensed pursuit costs		(4,772,000)	(772,000)	(4,957,000)	(882,000)
Income taxes	[1]	(64,000)	17,000	(240,000)	(1,203,000)
Other		(1,533,000)	(802,000)	(4,542,000)	(370,000)
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		24,176,000	0	53,920,000	0
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		18,910,000	17,292,000	55,192,000	49,226,000
Net operating income		13,621,000	12,198,000	39,698,000	34,257,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		16,147,000	23,417,000	47,789,000	66,848,000
Net operating income		9,554,000	12,519,000	28,913,000	35,761,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,072,000	28,115,000	43,802,000	85,230,000
Net operating income		10,176,000	18,340,000	29,686,000	53,129,000
Houston [Member]
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Impairment charges - real estate assets		(9,700,000)		(9,700,000)
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,167,000	14,870,000	44,720,000	43,840,000
Net operating income		8,701,000	7,989,000	23,263,000	24,858,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		40,698,000	34,157,000	122,048,000	102,182,000
Net operating income		29,995,000	24,982,000	88,656,000	73,939,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		11,137,000	10,513,000	33,501,000	31,253,000
Net operating income		8,743,000	7,750,000	25,987,000	23,089,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,422,000	24,563,000	45,165,000	74,789,000
Net operating income		9,246,000	16,244,000	25,866,000	44,544,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,773,000	3,275,000	10,204,000	10,352,000
Net operating income		1,781,000	2,031,000	5,288,000	6,871,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		33,288,000	34,831,000	95,826,000	104,274,000
Net operating income		$ 21,901,000	$ 23,201,000	$ 64,089,000	$ 68,980,000

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