Segment Information - Income Statement (Details) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Operating revenue		$ 176,604,000	$ 163,458,000	$ 359,198,000	$ 320,282,000
Net operating income		(22,733,000)	54,490,000	32,423,000	105,373,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense		(21,779,000)	(20,657,000)	(43,645,000)	(41,421,000)
Development service fee income		19,824,000	18,259,000	46,176,000	29,744,000
Development service fee expense		(79,808,000)	(17,828,000)	(107,875,000)	(28,832,000)
Depreciation and amortization expense		(42,708,000)	(41,682,000)	(84,781,000)	(83,162,000)
Impairment charges - real estate assets		26,000,000	0	26,000,000	0
Equity in earnings of unconsolidated joint ventures		7,428,000	3,990,000	14,192,000	9,721,000
General and administrative expense		(11,659,000)	(15,203,000)	(30,285,000)	(32,196,000)
Expensed pursuit costs		(59,000)	(153,000)	(383,000)	(185,000)
Systems implementation expense		(1,469,000)	(15,000)	(2,175,000)	(15,000)
Income taxes		(941,000)	(324,000)	(1,495,000)	(946,000)
Income from continuing operations		13,220,000	45,302,000	59,320,000	83,190,000
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue		0	0	0	0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue		169,483,000	164,551,000	341,573,000	328,631,000
Net operating income		115,916,000	110,518,000	234,068,000	217,730,000
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue		(291,000)	320,000	(389,000)	121,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[1]	(23,202,000)	(21,942,000)	(46,661,000)	(44,285,000)
Development service fee income		19,824,000	18,259,000	46,176,000	29,744,000
Development service fee expense		(79,808,000)	(17,828,000)	(107,875,000)	(28,832,000)
Depreciation and amortization expense	[1],[2]	(31,932,000)	(33,115,000)	(65,340,000)	(66,308,000)
Impairment charges - real estate assets		26,000,000	0	26,000,000	0
Gain on property dispositions		48,582,000	5,895,000	52,703,000	6,702,000
Equity in earnings of unconsolidated joint ventures		7,428,000	3,990,000	14,192,000	9,721,000
General and administrative expense	[1],[2]	(7,036,000)	(10,879,000)	(21,722,000)	(23,101,000)
Expensed pursuit costs		(59,000)	(153,000)	(383,000)	(185,000)
Systems implementation expense		(1,469,000)	(15,000)	(2,175,000)	(15,000)
Discontinued operations excluding (loss) gain on property dispositions		(7,908,000)	(7,516,000)	(15,484,000)	(13,872,000)
Income taxes	[2]	(6,000)	104,000	(18,000)	(176,000)
Other		(1,110,000)	(2,016,000)	(2,161,000)	(3,933,000)
Discontinued operations | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		(12,412,000)	(19,672,000)	(28,162,000)	(38,214,000)
Development Service Fee Income [Member] | Segment Reconciling Items [Member]
Segment Reporting Information [Line Items]
Operating revenue		19,824,000	18,259,000	46,176,000	29,744,000
Carolinas Richmond [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		19,513,000	18,225,000	39,306,000	36,282,000
Net operating income		14,271,000	13,094,000	28,318,000	26,077,000
Chicago Minneapolis [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		16,622,000	15,767,000	33,139,000	31,642,000
Net operating income		10,223,000	10,011,000	20,125,000	19,360,000
Florida [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		15,862,000	14,482,000	31,270,000	28,730,000
Net operating income		10,768,000	10,003,000	21,586,000	19,510,000
Houston [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		16,091,000	14,779,000	31,954,000	29,553,000
Net operating income		9,114,000	7,761,000	18,711,000	14,562,000
Lehigh Central PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		38,525,000	39,815,000	76,610,000	81,350,000
Net operating income		28,348,000	29,083,000	56,115,000	58,661,000
Philadelphia [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		10,717,000	10,928,000	23,061,000	22,363,000
Net operating income		8,566,000	8,475,000	18,720,000	17,244,000
Southeastern PA [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		6,694,000	15,127,000	16,876,000	29,742,000
Net operating income		4,516,000	8,507,000	11,405,000	16,619,000
United Kingdom [Member] | Operating Segments [Member] | UNITED KINGDOM
Segment Reporting Information [Line Items]
Operating revenue		3,783,000	3,262,000	8,056,000	6,431,000
Net operating income		1,577,000	1,649,000	4,006,000	3,506,000
Other Segments [Member] | Operating Segments [Member] | North America [Member]
Segment Reporting Information [Line Items]
Operating revenue		41,676,000	32,166,000	81,301,000	62,538,000
Net operating income		$ 28,533,000	$ 21,935,000	$ 55,082,000	$ 42,191,000

[1] Includes activity on discontinued operations.

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