Consolidated Statement of Changes in Equity (USD $) In Thousands	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]
Beginning balance at Dec. 31, 2010	$ 1,744,402	$ 1,744,402	$ 275,000	$ 819	$ (16,175)	$ 2,039,612	$ (80,885)	$ (80,885)	$ (533,194)	$ 1,685,177	$ 49,158	$ (762)	$ 10,829	$ 10,829	$ 59,225	$ 49,158	$ 1,766,062	$ (762)	$ 1,733,573
Net income	27,067	27,067	0	0	0	0	0	0	24,884	24,884	1,862	50	271	271	2,183	1,862	24,884	50	26,796
Other comprehensive income (loss)	4,733	4,733	0	0	0	0	4,723	4,723	0	4,723	0	10	0	0	10	0	0	10	4,733
Deferred compensation plan, net	1,905	1,905	0	0	1,190	715	0	0	0	1,905	0	0	0	0	0	0	1,905	0	1,905
Amortization of restricted stock issued	5,391		0	0	0	5,391	0		0	5,391	0	0	0		0
Common stock redeemed for taxes withheld for stock based compensation, net	(1,824)		0	0	0	(1,824)	0		0	(1,824)	0	0	0		0
Common stock issued for dividend reinvestment plan	562		0	0	0	562	0		0	562	0	0	0		0
Common stock issued for stock offerings, net of issuance costs	215,369		0	80	0	215,289	0		0	215,369	0	0	0		0
Contributions from partners	2,509	2,509	0	0	0	0	0	0	0	0	0	0	2,509	2,509	2,509	0	0	0	0
Partners' Capital Account, Distributions	810	79,720	0	0	0	0	0	0	0	0	0	0	810	810	810	0	78,750	160	78,910
Distributions to partners	(810)	(79,720)	0	0	0	0	0	0	0	0	0	0	(810)	(810)	(810)	0	(78,750)	(160)	(78,910)
Preferred unit distributions		(11,700)						0						0		(1,862)	(9,838)	0	(11,700)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company		5,391						0						0		0	5,391	0	5,391
Common units issued as a result of common stock issued by Parent Company, net of repurchases		214,107						0						0		0	214,107	0	214,107
Preferred stock/unit	(11,700)		0	0	0	0	0		(9,838)	(9,838)	(1,862)	0	0		(1,862)
Common stock/unit ($.925 per share)	(78,910)		0	0	0	0	0		(78,750)	(78,750)	0	(160)	0		(160)
Ending Balance at Jun. 30, 2011	1,908,694	1,908,694	275,000	899	(14,985)	2,259,745	(76,162)	(76,162)	(596,898)	1,847,599	49,158	(862)	12,799	12,799	61,095	49,158	1,923,761	(862)	1,895,895
Beginning balance at Mar. 31, 2011
Net income	18,937	18,937
Other comprehensive income (loss)	2,366	2,366
Ending Balance at Jun. 30, 2011	1,908,694	1,908,694
Beginning balance at Dec. 31, 2011	1,869,654	1,869,654	275,000	899	(15,197)	2,281,817	(71,429)	(71,429)	(662,735)	1,808,355	49,158	(963)	13,104	13,104	61,299	49,158	1,879,784	(963)	1,856,550
Net income	38,083	38,083	0	0	0	0	0	0	38,211	38,211	(629)	77	424	424	(128)	(629)	38,211	77	37,659
Other comprehensive income (loss)	4,682	4,682	0	0	0	0	4,698	4,698	0	4,698	0	9	(25)	(25)	(16)	0	0	9	4,707
Deferred compensation plan, net	0		0	0	405	(405)	0		0	0	0	0	0		0
Amortization of restricted stock issued	5,726		0	0	0	5,726	0		0	5,726	0	0	0		0
Common stock redeemed for taxes withheld for stock based compensation, net	(1,548)		0	0	0	(1,548)	0		0	(1,548)	0	0	0		0
Common stock issued for dividend reinvestment plan	495		0	0	0	495	0		0	495	0	0	0		0
Redemption of preferred units	(48,125)	(248,125)	0	0	0	0	0	0	0	0	(48,125)	0	0	0	(48,125)	(48,125)	(200,000)	0	(248,125)
Common stock issued for stock offerings, net of issuance costs	241,386	241,386	250,000	0	0	(8,614)	0	0	0	241,386	0	0	0	0	0	0	241,386	0	241,386
Stock Redeemed or Called During Period, Value	(200,000)		(200,000)	0	0	6,993	0		(6,993)	(200,000)	0	0	0		0
Contributions from partners	3,317	3,317	0	0	0	0	0	0	0	0	0	0	3,317	3,317	3,317	0	0	0	0
Partners' Capital Account, Distributions	576	83,327	0	0	0	0	0	0	0	0	0	0	576	576	576	0	82,587	164	82,751
Distributions to partners	(576)	(83,327)	0	0	0	0	0	0	0	0	0	0	(576)	(576)	(576)	0	(82,587)	(164)	(82,751)
Preferred unit distributions		(12,744)						0						0		(404)	(12,340)	0	(12,744)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company		5,726						0						0		0	5,726	0	5,726
Common units issued as a result of common stock issued by Parent Company, net of repurchases		(1,053)						0						0		0	(1,053)	0	(1,053)
Preferred stock/unit	(12,744)		0	0	0	0	0		(12,340)	(12,340)	(404)	0	0		(404)
Common stock/unit ($.925 per share)	(82,751)		0	0	0	0	0		(82,587)	(82,587)	0	(164)	0		(164)
Ending Balance at Jun. 30, 2012	1,817,599	1,817,599	325,000	899	(14,792)	2,284,464	(66,731)	(66,731)	(726,444)	1,802,396	0	(1,041)	16,244	16,244	15,203	0	1,869,127	(1,041)	1,801,355
Beginning balance at Mar. 31, 2012
Net income	11,348	11,348
Other comprehensive income (loss)	2,347	2,347
Ending Balance at Jun. 30, 2012	$ 1,817,599	$ 1,817,599