Consolidated Statement of Changes in Equity (USD $)	Parent Company [Member]	Partnership Interest [Member]	Preferred Stock [Member] Parent Company [Member]	Common Stock [Member] Parent Company [Member]	Treasury Stock [Member] Parent Company [Member]	Additional Paid In Capital [Member] Parent Company [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Parent Company [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Partnership Interest [Member]	Distributions in Excess of Net Income [Member] Parent Company [Member]	Total Stockholders' Equity [Member] Parent Company [Member]	Noncontrolling Interests Preferred Units [Member] Parent Company [Member]	Noncontrolling Interest Exchangeable Operating Partnership Units [Member] Parent Company [Member]	Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] Parent Company [Member]	Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] Partnership Interest [Member]	Total Noncontrolling Interests [Member] Parent Company [Member]	Preferred Units [Member] Partnership Interest [Member]	General Partner Preferred and Common Units [Member] Partnership Interest [Member]	Limited Partners [Member] Partnership Interest [Member]	Total Partners' Capital [Member] Partnership Interest [Member]	Maximum
Beginning balance at Dec. 31, 2010	$ 1,744,402,000	$ 1,744,402,000	$ 275,000,000	$ 819,000	$ (16,175,000)	$ 2,039,612,000	$ (80,885,000)	$ (80,885,000)	$ (533,194,000)	$ 1,685,177,000	$ 49,158,000	$ (762,000)	$ 10,829,000	$ 10,829,000	$ 59,225,000	$ 49,158,000	$ 1,766,062,000	$ (762,000)	$ 1,733,573,000
Net income	41,608,000	41,608,000	0	0	0	0	0	0	38,312,000	38,312,000	2,794,000	77,000	425,000	425,000	3,296,000	2,794,000	38,312,000	77,000	41,183,000
Other comprehensive income (loss)	7,099,000	7,099,000	0	0	0	0	7,085,000	7,085,000	0	7,085,000	0	14,000	0	0	14,000	0	0	14,000	7,099,000
Deferred compensation plan, net	17,843,000	17,843,000	0	0	1,147,000	16,696,000	0	0	0	17,843,000	0	0	0	0	0	0	17,843,000	0	17,843,000
Common stock redeemed for taxes withheld for stock based compensation, net	(1,752,000)		0	0	0	(1,752,000)	0		0	(1,752,000)	0	0	0		0
Common stock issued for dividend reinvestment plan	821,000		0	0	0	821,000	0		0	821,000	0	0	0		0
Common stock issued for stock offerings, net of issuance costs	215,369,000		0	80,000	0	215,289,000	0		0	215,369,000	0	0	0		0
Contributions from partners	2,646,000	2,646,000	0	0	0	0	0	0	0	0	0	0	2,646,000	2,646,000	2,646,000	0	0	0	0
Distributions to partners	(865,000)	(121,099,000)	0	0	0	0	0	0	0	0	0	0	(865,000)	(865,000)	(865,000)	0	(119,988,000)	(246,000)	(120,234,000)
Preferred unit distributions		(17,550,000)						0						0		(2,794,000)	(14,756,000)	0	(17,550,000)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company	8,087,000	8,087,000	0	0	0	8,087,000	0	0	0	8,087,000	0	0	0	0	0	0	8,087,000	0	8,087,000
Common units issued as a result of common stock issued by Parent Company, net of repurchases		214,438,000						0						0		0	214,438,000	0	214,438,000
Preferred stock/unit	(17,550,000)		0	0	0	0	0		(14,756,000)	(14,756,000)	(2,794,000)	0	0		(2,794,000)
Common stock/unit ($1.3875 per share)	(120,234,000)		0	0	0	0	0		(119,988,000)	(119,988,000)	0	(246,000)	0		(246,000)
Ending Balance at Sep. 30, 2011	1,897,474,000	1,897,474,000	275,000,000	899,000	(15,028,000)	2,278,753,000	(73,800,000)	(73,800,000)	(629,626,000)	1,836,198,000	49,158,000	(917,000)	13,035,000	13,035,000	61,276,000	49,158,000	1,909,998,000	(917,000)	1,884,439,000
Beginning balance at Jun. 30, 2011
Net income	14,541,000	14,541,000
Other comprehensive income (loss)	2,366,000	2,366,000
Ending Balance at Sep. 30, 2011	1,897,474,000	1,897,474,000
Beginning balance at Dec. 31, 2011	1,869,654,000	1,869,654,000	275,000,000	899,000	(15,197,000)	2,281,817,000	(71,429,000)	(71,429,000)	(662,735,000)	1,808,355,000	49,158,000	(963,000)	13,104,000	13,104,000	61,299,000	49,158,000	1,879,784,000	(963,000)	1,856,550,000
Stock Issued During Period, Value, Other																				128,000,000
Net income	57,903,000	57,903,000	0	0	0	0	0	0	57,780,000	57,780,000	(629,000)	116,000	636,000	636,000	123,000	(629,000)	57,780,000	116,000	57,267,000
Other comprehensive income (loss)	7,031,000	7,031,000	0	0	0	0	7,051,000	7,051,000	0	7,051,000	0	14,000	(34,000)	(34,000)	(20,000)	0	0	14,000	7,065,000
Deferred compensation plan, net	12,000	(12,000)	0	0	441,000	(429,000)	0	0	0	12,000	0	0	0	0	0	0	(12,000)	0	(12,000)
Common stock redeemed for taxes withheld for stock based compensation, net	(1,490,000)		0	0	0	(1,490,000)	0		0	(1,490,000)	0	0	0		0
Common stock issued for dividend reinvestment plan	740,000		0	0	0	740,000	0		0	740,000	0	0	0		0
Redemption of preferred units	(48,125,000)	(48,125,000)	0	0	0	0	0	0	0		(48,125,000)	0	0	0	(48,125,000)	(48,125,000)	0	0	(48,125,000)
Preferred Stock, Issued During Period, Value	313,505,000		325,000,000	0	0	(11,495,000)	0		0	313,505,000	0	0	0		0
Common stock issued for stock offerings, net of issuance costs	21,677,000	313,505,000	0	5,000	0	21,672,000	0	0	0	21,677,000	0	0	0	0	0	0	313,505,000	0	313,505,000
Stock Redeemed or Called During Period, Value	(275,000,000)	(275,000,000)	(275,000,000)	0	0	9,277,000	0		(9,277,000)	(275,000,000)	0	0	0		0		(275,000,000)		(275,000,000)
Contributions from partners	3,362,000	3,362,000	0	0	0	0	0	0	0	0	0	0	3,362,000	3,362,000	3,362,000	0	0	0	0
Distributions to partners	(892,000)	(125,031,000)	0	0	0	0	0	0	0	0	0	0	(892,000)	(892,000)	(892,000)	0	(123,893,000)	(246,000)	(124,139,000)
Preferred unit distributions		(18,392,000)						0						0		(404,000)	(17,988,000)	0	(18,392,000)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company	8,589,000	8,589,000	0	0	0	8,589,000	0	0	0	8,589,000	0	0	0	0	0	0	8,589,000	0	8,589,000
Common units issued as a result of common stock issued by Parent Company, net of repurchases		20,926,000						0						0		0	20,926,000	0	20,926,000
Preferred stock/unit	(18,392,000)		0	0	0	0	0		(17,988,000)	(17,988,000)	(404,000)	0	0		(404,000)
Common stock/unit ($1.3875 per share)	(124,140,000)		0	0	0	0	0		(123,894,000)	(123,894,000)	0	(246,000)	0		(246,000)
Ending Balance at Sep. 30, 2012	1,814,434,000	1,814,434,000	325,000,000	904,000	(14,756,000)	2,308,681,000	(64,378,000)	(64,378,000)	(756,114,000)	1,799,337,000	0	(1,079,000)	16,176,000	16,176,000	15,097,000	0	1,863,715,000	(1,079,000)	1,798,258,000
Beginning balance at Jun. 30, 2012
Stock Issued During Period, Value, Other																				128,000,000
Net income	19,820,000	19,820,000
Other comprehensive income (loss)	2,349,000	2,349,000
Ending Balance at Sep. 30, 2012	$ 1,814,434,000	$ 1,814,434,000