Quarterly Financial Data (Unaudited) (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Unaudited Quarterly Financial Data [Line Items]
Total revenues		$ 544,674,000	$ 525,630,000	$ 505,761,000	$ 492,350,000	$ 482,852,000	$ 462,881,000	$ 445,408,000
Total revenues subsequently reclassified to discontinued operations		(11,470,000)	(18,151,000)	(21,898,000)	(21,777,000)	(11,020,000)	(17,486,000)	(20,948,000)
Operating income		179,867,000	158,988,000	140,889,000	154,722,000	142,781,000	134,130,000	110,042,000	667,958,000	541,675,000	348,210,000
Operating income subsequently reclassified to discontinued operations		(4,260,000)	(6,723,000)	(5,263,000)	(5,231,000)	(2,033,000)	(5,727,000)	(5,911,000)
(Loss) income from continuing operations		128,433,000	30,213,000	12,654,000	32,506,000	28,274,000	10,336,000	(13,322,000)	311,555,000	57,794,000	(103,108,000)
Income from continuing operations subsequently reclassified to discontinued operations		(4,248,000)	(6,345,000)	(4,735,000)	(4,265,000)	(1,943,000)	(4,790,000)	(4,409,000)
Discontinued operations, net		107,890,000	78,102,000	139,513,000	74,895,000	84,703,000	571,417,000	146,388,000	569,649,000	877,403,000	399,091,000
Discontinued operations, net from properties sold subsequent to the respective reporting period		4,248,000	6,345,000	4,735,000	4,265,000	1,943,000	4,790,000	4,409,000
Net income									881,204,000	935,197,000	295,983,000
Net income available to Common Shares	365,979,000	218,603,000	99,797,000	141,833,000	99,016,000	104,382,000	552,457,000	123,865,000	826,212,000	879,720,000	269,242,000
Net income available to Common Shares	$ 1.18	$ 0.73	$ 0.33	$ 0.47	$ 0.33	$ 0.35	$ 1.88	$ 0.42	$ 2.729	$ 2.984	$ 0.952
Weighted Average Number of Shares Outstanding, Basic	310,398	301,336	300,193	298,805	295,990	295,831	294,663	292,895	302,701	294,856	282,888
Net income per share - diluted	$ 1.17	$ 0.72	$ 0.33	$ 0.47	$ 0.33	$ 0.35	$ 1.85	$ 0.42	$ 2.705	$ 2.950	$ 0.952
Weighted average Common Shares outstanding	327,108	318,773	317,648	315,230	312,731	312,844	312,199	292,895	319,766	312,065	282,888
Weighted average Units outstanding									316,554	308,062	296,527
Weighted average Units outstanding									319,766	312,065	296,527
Operating Partnership [Member]
Unaudited Quarterly Financial Data [Line Items]
Total revenues	547,650,000	544,674,000	525,630,000	505,761,000	492,350,000	482,852,000	462,881,000	445,408,000
Operating income	188,214,000	179,867,000	158,988,000	140,889,000	154,722,000	142,781,000	134,130,000	110,042,000
(Loss) income from continuing operations	140,255,000	128,433,000	30,213,000	12,654,000	32,506,000	28,274,000	10,336,000	(13,322,000)
Discontinued operations, net	244,144,000	107,890,000	78,102,000	139,513,000	74,895,000	84,703,000	571,417,000	146,388,000
Net income	384,399,000	236,323,000	108,315,000	152,167,000	107,401,000	112,977,000	581,753,000	133,066,000
Net income available to Units	382,974,000	229,099,000	104,529,000	148,251,000	103,521,000	109,124,000	578,215,000	129,640,000
Net income available to Common Shares	$ 1.18	$ 0.73	$ 0.33	$ 0.47	$ 0.33	$ 0.35	$ 1.88	$ 0.42
Net income per share - diluted	$ 1.17	$ 0.72	$ 0.33	$ 0.47	$ 0.33	$ 0.35	$ 1.85	$ 0.42
Weighted average Units outstanding	324,364	315,513	314,255	312,011	309,120	308,884	307,954	306,248
Weighted average Units outstanding	327,108	318,773	317,648	315,230	312,731	312,844	312,199	306,248
Scenario, Previously Reported [Member]
Unaudited Quarterly Financial Data [Line Items]
Total revenues		556,144,000	543,781,000	527,659,000	514,127,000	493,872,000	480,367,000	466,356,000
Operating income		184,127,000	165,711,000	146,152,000	159,953,000	144,814,000	139,857,000	115,953,000
(Loss) income from continuing operations		132,681,000	36,558,000	17,389,000	36,771,000	30,217,000	15,126,000	(8,913,000)
Discontinued operations, net		$ 103,642,000	$ 71,757,000	$ 134,778,000	$ 70,630,000	$ 82,760,000	$ 566,627,000	$ 141,979,000