Segment Information (Performance of the Reportable Segments Based on Property Level Operating Income) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total revenue	$ 180,281	$ 170,389	$ 174,091	$ 180,128	$ 179,825	$ 170,783	$ 160,747	$ 154,029	$ 704,889	$ 665,384	$ 675,323
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(89,630)	(84,238)	(105,127)
Loss on debt extinguishment									0	(49)	(27,099)
Development service fee income									73,224	82,673	12,941
Development service fee expense									(134,825)	(85,805)	(12,165)
Depreciation and amortization expense									(169,717)	(162,700)	(191,448)
Impairment - real estate assets									(32,431)	(3,946)	(3,879)
Gain on property dispositions									81,514	100,387	219,270
Equity in earnings of unconsolidated joint ventures									21,382	17,155	21,970
General and administrative expense									(53,843)	(56,191)	(63,071)
Other operating expenses									(1,795)	(5,066)	(1,016)
Income taxes									(7,364)	(1,992)	(1,969)
Income from continuing operations	$ 58,084	$ 49,740	$ 12,942	$ 45,299	$ 109,245	$ 57,770	$ 44,460	$ 37,331	166,065	248,806	339,016
Operating Segments
Segment Reporting Information [Line Items]
Total revenue									687,851	671,280	734,050
Net operating income									470,311	448,521	468,786
Operating Segments | Carolinas/Richmond
Segment Reporting Information [Line Items]
Total revenue									79,271	74,158	66,270
Net operating income									57,105	53,132	46,170
Operating Segments | Chicago/Minneapolis
Segment Reporting Information [Line Items]
Total revenue									67,551	64,706	84,028
Net operating income									40,781	39,231	45,258
Operating Segments | Florida
Segment Reporting Information [Line Items]
Total revenue									64,486	59,526	99,733
Net operating income									44,204	39,850	62,177
Operating Segments | Houston
Segment Reporting Information [Line Items]
Total revenue									64,968	59,957	59,582
Net operating income									37,766	32,300	32,499
Operating Segments | Lehigh/Central PA
Segment Reporting Information [Line Items]
Total revenue									156,629	163,026	141,046
Net operating income									114,201	119,304	102,209
Operating Segments | Philadelphia
Segment Reporting Information [Line Items]
Total revenue									44,387	45,504	41,838
Net operating income									36,216	35,279	30,862
Operating Segments | Southeastern PA
Segment Reporting Information [Line Items]
Total revenue									27,874	59,762	91,656
Net operating income									20,044	35,641	53,947
Operating Segments | Southern California
Segment Reporting Information [Line Items]
Total revenue									30,939	13,856	11,019
Net operating income									24,360	10,199	7,079
Operating Segments | United Kingdom
Segment Reporting Information [Line Items]
Total revenue									17,193	13,917	13,376
Net operating income									8,996	7,657	6,390
Operating Segments | Other
Segment Reporting Information [Line Items]
Total revenue									134,553	116,868	125,502
Net operating income									86,638	75,928	82,195
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Total revenue									(130)	(140)	(283)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(95,442)	(91,096)	(115,077)
Loss on debt extinguishment										(49)	(27,099)
Development service fee income									73,224	82,673	12,941
Development service fee expense									(134,825)	(85,805)	(12,165)
Depreciation and amortization expense									(129,048)	(132,613)	(152,556)
Impairment - real estate assets									(39,688)	(10,632)	(3,879)
Gain on property dispositions									81,514	100,387	219,270
Equity in earnings of unconsolidated joint ventures									21,382	17,155	21,970
General and administrative expense									(37,278)	(39,024)	(40,090)
Other operating expenses									(10,835)	(5,294)	(4,418)
Discontinued operations excluding gain on property dispositions									(23,379)	(26,661)	(27,129)
Income taxes									(4,701)	(240)	47
Other									$ (5,170)	$ (8,516)	$ (1,585)