Segment Information (Performance of the Reportable Segments Based on Property Level Operating Income) (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Operating revenue		$ 192,803,000	$ 184,511,000	$ 174,888,000	$ 167,576,000	$ 169,733,000	$ 181,967,000	$ 178,033,000	$ 180,965,000	$ 719,778,000	$ 710,698,000	$ 773,960,000
Development service fee income	[1]									82,673,000	12,941,000	0
Operating Income (Loss)										236,644,000	240,466,000	244,368,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense										(88,857,000)	(112,299,000)	(133,557,000)
Loss on debt extinguishment										(49,000)	(27,099,000)	0
Depreciation, Depletion and Amortization, Nonproduction										175,137,000	196,705,000	217,864,000
Impairment - real estate assets (2)										(3,946,000)	(3,879,000)	(17,215,000)
Gain on property dispositions										100,387,000	219,270,000	100,314,000
Equity in earnings of unconsolidated joint ventures										17,155,000	21,970,000	3,149,000
General and administrative expense										(56,975,000)	(67,088,000)	(68,264,000)
Expensed pursuit costs										(5,066,000)	(1,016,000)	(605,000)
Income taxes										(1,992,000)	(1,971,000)	(2,673,000)
Income from continuing operations		$ 114,463,000	$ 63,968,000	$ 50,510,000	$ 42,125,000	$ 197,038,000	$ 51,675,000	$ 48,815,000	$ 56,626,000	271,066,000	354,154,000	235,359,000
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										671,280,000	734,050,000	808,683,000
Operating Income (Loss)										448,521,000	468,786,000	504,774,000
Segment Reconciling Items [Member] | Discontinued Operations [Member]
Segment Reporting Information [Line Items]
Operating revenue										(34,035,000)	(36,010,000)	(34,813,000)
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Operating revenue										(140,000)	(283,000)	90,000
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items]
Interest expense	[2]									(91,096,000)	(115,077,000)	(135,779,000)
Loss on debt extinguishment											(27,099,000)	0
Depreciation, Depletion and Amortization, Nonproduction	[2],[3]									132,613,000	152,556,000
Depreciation and amortization expense	[2],[3]											(166,500,000)
Impairment - real estate assets (2)	[2]									(10,632,000)	(3,879,000)	(18,244,000)
Gain on property dispositions										100,387,000
Gain on property dispositions											219,270,000	100,314,000
Equity in earnings of unconsolidated joint ventures										17,155,000	21,970,000	3,149,000
General and administrative expense	[2],[3]									(45,191,000)	(44,341,000)	(43,924,000)
Expensed pursuit costs										(5,006,000)	(1,016,000)	(605,000)
Discontinued operations excluding gain on property dispositions										(4,401,000)	(11,991,000)	(10,565,000)
Income taxes	[3]									(240,000)	47,000	(2,351,000)
Other										5,769,000	(40,000)	(5,090,000)
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										74,158,000	66,270,000	71,950,000
Operating Income (Loss)										53,132,000	46,170,000	47,849,000
North America [Member] | Operating Segments [Member] | Chicago Minneapolis [Member]
Segment Reporting Information [Line Items]
Operating revenue										64,706,000	84,028,000	85,363,000
Operating Income (Loss)										39,231,000	45,258,000	44,963,000
North America [Member] | Operating Segments [Member] | Florida [Member]
Segment Reporting Information [Line Items]
Operating revenue										59,526,000	99,733,000	127,184,000
Operating Income (Loss)										39,850,000	62,177,000	77,283,000
North America [Member] | Operating Segments [Member] | Houston [Member]
Segment Reporting Information [Line Items]
Operating revenue										59,957,000	59,582,000	54,031,000
Operating Income (Loss)										32,300,000	32,499,000	31,159,000
North America [Member] | Operating Segments [Member] | Lehigh Central PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										163,026,000	141,046,000	133,473,000
Operating Income (Loss)										119,304,000	102,209,000	94,972,000
North America [Member] | Operating Segments [Member] | Philadelphia [Member]
Segment Reporting Information [Line Items]
Operating revenue										45,504,000	41,838,000	40,490,000
Operating Income (Loss)										35,279,000	30,862,000	29,679,000
North America [Member] | Operating Segments [Member] | Southeastern PA [Member]
Segment Reporting Information [Line Items]
Operating revenue										59,762,000	91,656,000	147,712,000
Operating Income (Loss)										35,641,000	53,947,000	80,458,000
North America [Member] | Operating Segments [Member] | Other Segments [Member]
Segment Reporting Information [Line Items]
Operating revenue										130,724,000	136,521,000	133,345,000
Operating Income (Loss)										86,127,000	89,274,000	87,925,000
United Kingdom [Member] | Operating Segments [Member] | United Kingdom [Member]
Segment Reporting Information [Line Items]
Operating revenue										13,917,000	13,376,000	15,135,000
Operating Income (Loss)										$ 7,657,000	$ 6,390,000	$ 10,486,000

[1] (1) Prior to the fourth quarter of 2016, development service fee income had been classified as other income and development service fee expense had been classified as general and administrative expense. See Note 2.

[2] (2) Includes activity on discontinued operations.

[3] (3) Excludes costs which are included in determining segment-level net operating income.