Segment Information - Income Statement (Details) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Revenues		$ 171,184,000	$ 171,587,000	$ 527,046,000	$ 487,965,000
Net operating income		61,645,000	50,185,000	93,059,000	154,161,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(23,274,000)	(21,448,000)	(66,582,000)	(62,409,000)
Development service fee income		12,956,000	24,176,000	59,132,000	53,920,000
Development service fee expense		(12,924,000)	(23,665,000)	(120,799,000)	(52,497,000)
Depreciation and amortization expense		(43,375,000)	(41,754,000)	(127,280,000)	(123,924,000)
Impairment charges - real estate assets		0	(3,936,000)	(26,000,000)	(3,936,000)
Gain on property dispositions		2,002,000	23,840,000	54,705,000	30,542,000
Equity in earnings of unconsolidated joint ventures		6,766,000	4,305,000	20,958,000	14,026,000
General and administrative expense		(13,121,000)	(11,873,000)	(43,406,000)	(44,069,000)
Expensed pursuit costs		(1,292,000)	(4,772,000)	(1,675,000)	(4,957,000)
Systems implementation expense		(1,177,000)	(142,000)	(3,352,000)	(157,000)
Income taxes		(472,000)	(582,000)	(1,967,000)	(1,528,000)
Income from continuing operations		50,100,000	58,034,000	108,741,000	140,279,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Revenues		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Revenues		172,223,000	169,614,000	513,797,000	498,247,000
Net operating income		120,257,000	113,718,000	354,325,000	331,446,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues		18,000	(137,000)	(372,000)	(18,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(24,670,000)	(23,060,000)	(71,331,000)	(67,345,000)
Development service fee income		12,956,000	24,176,000	59,132,000	53,920,000
Development service fee expense		(12,924,000)	(23,665,000)	(120,799,000)	(52,497,000)
Depreciation and amortization expense	[1],[2]	(32,243,000)	(33,521,000)	(97,582,000)	(99,829,000)
Impairment charges - real estate assets	[1]	0	(9,650,000)	(26,000,000)	(9,650,000)
Gain on property dispositions		2,002,000	23,840,000	54,705,000	30,542,000
Equity in earnings of unconsolidated joint ventures		6,766,000	4,305,000	20,958,000	14,026,000
General and administrative expense	[1],[2]	(9,615,000)	(7,962,000)	(30,954,000)	(31,073,000)
Expensed pursuit costs		(1,292,000)	(4,772,000)	(1,675,000)	(4,957,000)
Systems implementation expense		(1,177,000)	(142,000)	(3,352,000)	(157,000)
Discontinued operations excluding (loss) gain on property dispositions		(9,093,000)	(3,125,000)	(25,256,000)	(17,942,000)
Income taxes	[2]	(70,000)	(64,000)	(89,000)	(240,000)
Other		(797,000)	(2,044,000)	(3,341,000)	(5,965,000)
Discontinued operations | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Revenues		(14,013,000)	(22,066,000)	(45,511,000)	(64,184,000)
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Revenues		12,956,000	24,176,000	59,132,000	53,920,000
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		19,417,000	18,910,000	58,723,000	55,192,000
Net operating income		14,254,000	13,621,000	42,572,000	39,698,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		17,274,000	16,147,000	50,414,000	47,789,000
Net operating income		10,292,000	9,554,000	30,417,000	28,913,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		15,847,000	15,072,000	47,117,000	43,802,000
Net operating income		11,148,000	10,176,000	32,734,000	29,686,000
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		16,561,000	15,167,000	48,514,000	44,720,000
Net operating income		9,514,000	8,701,000	28,225,000	23,263,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		39,188,000	40,698,000	115,799,000	122,048,000
Net operating income		29,182,000	29,995,000	85,297,000	88,656,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		10,598,000	11,137,000	33,659,000	33,501,000
Net operating income		8,678,000	8,743,000	27,398,000	25,987,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		6,697,000	15,422,000	23,573,000	45,165,000
Net operating income		5,316,000	9,246,000	16,721,000	25,866,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Revenues		3,523,000	3,773,000	11,579,000	10,204,000
Net operating income		1,873,000	1,781,000	5,879,000	5,288,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Revenues		43,118,000	33,288,000	124,419,000	95,826,000
Net operating income		$ 30,000,000	$ 21,901,000	$ 85,082,000	$ 64,089,000

[1] Includes activity in discontinued operations.

[2] Excludes costs that are included in determining SNOI.