Segment Information (Performance of the Reportable Segments Based on Property Level Operating Income) (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Operating revenue	$ 179,057,000	$ 190,857,000	$ 186,654,000	$ 190,140,000	$ 199,382,000	$ 198,972,000	$ 203,518,000	$ 206,901,000	$ 746,708,000	$ 808,773,000	$ 792,631,000
Operating Income (Loss)									255,102,000	257,132,000	257,306,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(115,077,000)	(135,779,000)	(151,887,000)
Loss on debt extinguishment									(27,099,000)	0	0
Depreciation and amortization expense									(207,673,000)	(229,779,000)	(236,075,000)
Impairment - real estate assets									(3,879,000)	(18,244,000)	(117,000)
Gain on property dispositions									219,270,000	100,314,000	45,147,000
Equity in earnings of unconsolidated joint ventures									21,970,000	3,149,000	10,314,000
General and administrative expense									(68,198,000)	(68,710,000)	(63,327,000)
Income taxes									(1,971,000)	(2,673,000)	(2,590,000)
Income from continuing operations									366,145,000	245,924,000	175,890,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue									734,050,000	808,683,000	797,723,000
Operating Income (Loss)									468,786,000	504,774,000	493,234,000
Segment Reconciling Items [Member] | Development Service Fee Income [Member]
Segment Reporting Information [Line Items]
Operating revenue									12,941,000	0	0
Segment Reconciling Items [Member] | Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue									0	0	(4,728,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue									(283,000)	90,000	(364,000)
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense									(115,077,000)	(135,779,000)	(152,444,000)
Depreciation and amortization expense									(152,556,000)	(166,500,000)	(177,715,000)
Impairment - real estate assets									(3,879,000)	(18,244,000)	(117,000)
Gain on property dispositions									219,270,000	100,314,000	45,147,000
Equity in earnings of unconsolidated joint ventures									21,970,000	3,149,000	10,314,000
General and administrative expense									(45,357,000)	(44,529,000)	(38,041,000)
Discontinued operations excluding gain on property dispositions									0	0	(1,950,000)
Income taxes									47,000	(2,351,000)	(2,404,000)
Other									40,000	5,090,000	(134,000)
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenue									0
North America [Member] | Operating Segments [Member] | Carolinas Richmond [Member]
Segment Reporting Information [Line Items]
Operating revenue									66,270,000	71,950,000	76,781,000
Operating Income (Loss)									46,170,000	47,849,000	48,935,000
North America [Member] | Operating Segments [Member] | Chicago Milwaukee [Member]
Segment Reporting Information [Line Items]
Operating revenue									42,836,000	37,431,000	33,068,000
Operating Income (Loss)									28,199,000	24,851,000	20,856,000
North America [Member] | Operating Segments [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Operating revenue									99,733,000	127,184,000	135,203,000
Operating Income (Loss)									62,177,000	77,283,000	83,195,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Operating revenue									59,582,000	54,031,000	49,309,000
Operating Income (Loss)									32,499,000	31,159,000	29,134,000
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue									141,046,000	133,473,000	118,145,000
Operating Income (Loss)									102,209,000	94,972,000	82,050,000
North America [Member] | Operating Segments [Member] | Minnesota [Member]
Segment Reporting Information [Line Items]
Operating revenue									41,192,000	47,932,000	54,892,000
Operating Income (Loss)									17,059,000	20,112,000	26,464,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Operating revenue									41,838,000	40,490,000	35,238,000
Operating Income (Loss)									30,862,000	29,679,000	26,722,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue									91,656,000	147,712,000	148,714,000
Operating Income (Loss)									53,947,000	80,458,000	81,183,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue									136,521,000	133,345,000	130,013,000
Operating Income (Loss)									89,274,000	87,925,000	83,991,000
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue									13,376,000	15,135,000	16,360,000
Operating Income (Loss)									$ 6,390,000	$ 10,486,000	$ 10,704,000