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	8,130	8,130	0	0	0	0	0	0	7,104	7,104	931	13	82	82	1,026	931	7,104	13	8,048
Other comprehensive income (loss)	2,367	2,367	0	0	0	0	2,362	2,362	0	2,362	0	5	0	0	5	0	0	5	2,367
Deferred compensation plan, net	(24)	(24)	0	0	(117)	93	0	0	0	(24)	0	0	0	0	0	0	(24)	0	(24)
Amortization of restricted stock issued	3,288		0	0	0	3,288	0		0	3,288	0	0	0		0
Common stock redeemed for taxes withheld for stock based compensation, net	(1,895)		0	0	0	(1,895)	0		0	(1,895)	0	0	0		0
Common stock issued for dividend reinvestment plan	287		0	0	0	287	0		0	287	0	0	0		0
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	2,289	2,289	0	0	0	0	0	0	0	0	0	0	2,289	2,289	2,289	0	0	0	0
Partners' Capital Account, Distributions	292	37,891	0	0	0	0	0	0	0	0	0	0	292	292	292	0	37,517	82	37,599
Distributions to partners	(292)	(37,891)	0	0	0	0	0	0	0	0	0	0	(292)	(292)	(292)	0	(37,517)	(82)	(37,599)
Preferred unit distributions		(5,850)						0						0		(931)	(4,919)	0	(5,850)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company		3,288						0						0		0	3,288	0	3,288
Common units issued as a result of common stock issued by Parent Company, net of repurchases		213,761						0						0		0	213,761	0	213,761
Preferred stock/unit	(5,850)		0	0	0	0	0		(4,919)	(4,919)	(931)	0	0		(931)
Common stock/unit ($.4625 per share)	(37,599)		0	0	0	0	0		(37,517)	(37,517)	0	(82)	0		(82)
Ending Balance at Mar. 31, 2011	1,930,472	1,930,472	275,000	899	(16,292)	2,256,674	(78,523)	(78,523)	(568,526)	1,869,232	49,158	(826)	12,908	12,908	61,240	49,158	1,947,755	(826)	1,917,564
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	26,735	26,735	0	0	0	0	0	0	27,118	27,118	(629)	54	192	192	(383)	(629)	27,118	54	26,543
Other comprehensive income (loss)	2,336	2,336	0	0	0	0	2,346	2,346	0	2,346	0	5	(15)	(15)	(10)	0	0	5	2,351
Deferred compensation plan, net	0	0	0	0	975	(975)	0	0	0	0	0	0	0	0	0	0	0	0	0
Amortization of restricted stock issued	2,863		0	0	0	2,863	0		0	2,863	0	0	0		0
Common stock redeemed for taxes withheld for stock based compensation, net	(1,623)		0	0	0	(1,623)	0		0	(1,623)	0	0	0		0
Common stock issued for dividend reinvestment plan	256		0	0	0	256	0		0	256	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)
Redemption of preferred stock	(200,000)		(200,000)	0	0	6,993	0		(6,993)	(200,000)	0	0	0		0
Stock issued for stock offerings, net of issuance costs	241,450	241,450	250,000	0	0	(8,550)	0	0	0	241,450	0	0	0	0	0	0	241,450	0	241,450
Contributions from partners	42	42	0	0	0	0	0	0	0	0	0	0	42	42	42	0	0	0	0
Partners' Capital Account, Distributions	249	41,626	0	0	0	0	0	0	0	0	0	0	249	249	249	0	41,291	86	41,377
Distributions to partners	(249)	(41,626)	0	0	0	0	0	0	0	0	0	0	(249)	(249)	(249)	0	(41,291)	(86)	(41,377)
Preferred unit distributions		(7,348)						0						0		(404)	(6,944)	0	(7,348)
Restricted units issued as a result of amortization of restricted stock issued by Parent Company		2,863						0						0		0	2,863	0	2,863
Common units issued as a result of common stock issued by Parent Company, net of repurchases		(1,367)						0						0		0	(1,367)	0	(1,367)
Preferred stock/unit	(7,348)		0	0	0	0	0		(6,944)	(6,944)	(404)	0	0		(404)
Common stock/unit ($.4625 per share)	(41,377)		0	0	0	0	0		(41,291)	(41,291)	0	(86)	0		(86)
Ending Balance at Mar. 31, 2012	$ 1,844,614	$ 1,844,614	$ 325,000	$ 899	$ (14,222)	$ 2,280,781	$ (69,083)	$ (69,083)	$ (690,845)	$ 1,832,530	$ 0	$ (990)	$ 13,074	$ 13,074	$ 12,084	$ 0	$ 1,901,613	$ (990)	$ 1,831,540