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]	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]	Preferred Units [Member] Partnership Interest [Member]	Preferred Units [Member] Accumulated Other Comprehensive Income (Loss) [Member] Partnership Interest [Member]	Preferred Units [Member] Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] Partnership Interest [Member]	Preferred Units [Member] Partnership Interest [Member]	Preferred Units [Member] General Partner Preferred and Common Units [Member] Partnership Interest [Member]	Preferred Units [Member] Limited Partners [Member] Partnership Interest [Member]	Preferred Units [Member] Total Partners' Capital [Member] Partnership Interest [Member]	Preferred Stock [Member] Parent Company [Member]	Preferred Stock [Member] Parent Company [Member]	Preferred Stock [Member] Common Stock [Member] Parent Company [Member]	Preferred Stock [Member] Treasury Stock [Member] Parent Company [Member]	Preferred Stock [Member] Additional Paid In Capital [Member] Parent Company [Member]	Preferred Stock [Member] Accumulated Other Comprehensive Income (Loss) [Member] Parent Company [Member]	Preferred Stock [Member] Distributions in Excess of Net Income [Member] Parent Company [Member]	Preferred Stock [Member] Total Stockholders' Equity [Member] Parent Company [Member]	Preferred Stock [Member] Noncontrolling Interests Preferred Units [Member] Parent Company [Member]	Preferred Stock [Member] Noncontrolling Interest Exchangeable Operating Partnership Units [Member] Parent Company [Member]	Preferred Stock [Member] Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] Parent Company [Member]	Preferred Stock [Member] Total Noncontrolling Interests [Member] Parent Company [Member]	Common Stock [Member] Parent Company [Member]	Common Stock [Member] Preferred Stock [Member] Parent Company [Member]	Common Stock [Member] Parent Company [Member]	Common Stock [Member] Treasury Stock [Member] Parent Company [Member]	Common Stock [Member] Additional Paid In Capital [Member] Parent Company [Member]	Common Stock [Member] Accumulated Other Comprehensive Income (Loss) [Member] Parent Company [Member]	Common Stock [Member] Distributions in Excess of Net Income [Member] Parent Company [Member]	Common Stock [Member] Total Stockholders' Equity [Member] Parent Company [Member]	Common Stock [Member] Noncontrolling Interests Preferred Units [Member] Parent Company [Member]	Common Stock [Member] Noncontrolling Interest Exchangeable Operating Partnership Units [Member] Parent Company [Member]	Common Stock [Member] Noncontrolling Interests in Limited Partners' Interest in Consolidated Partnerships [Member] Parent Company [Member]	Common Stock [Member] Total Noncontrolling Interests [Member] Parent Company [Member]
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	975	(975)		0		0	0	0	0	0		0
Amortization of restricted stock issued	2,863	2,863	0	0	0	2,863		0	0	0	2,863	0	0	0	0	0	0	2,863	0	2,863
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)		0	0	0	0		0		0	0	(48,125)	0	0		(48,125)
Preferred unit distributions		(7,348)							0						0		(404)	(6,944)	0	(7,348)
Stock issued for stock offerings, net of issuance costs		241,450							0						0		0	241,450	0	241,450								241,450	250,000	0	0	(8,550)	0	0	241,450	0	0	0	0
Redemption of preferred stock	(200,000)		(200,000)	0	0	6,993		0		(6,993)	(200,000)	0	0	0		0
Contributions from partners	42	42	0	0	0	0		0	0	0	0	0	0	42	42	42	0	0	0	0
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 stock redemptions																					(248,125)	0	0	(48,125)	(200,000)	0	(248,125)
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	(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
Beginning balance at Dec. 31, 2012	1,745,911	1,745,911	325,000	904	(14,924)	2,312,310		(57,715)	(57,715)	(834,810)	1,730,765	0	(1,153)	16,299	16,299	15,146	0	1,788,480	(1,153)	1,729,612
Net income	21,134	21,134	0	0	0	0		0	0	20,820	20,820	0	39	275	275	314	0	20,820	39	20,859
Other comprehensive income (loss)	5,747	5,747	0	0	0	0	5,732	5,732	5,732	0	5,732	0	11	4	4	15	0	0	11	5,743
Deferred compensation plan, net	0		0	0	(988)	988		0		0	0	0	0	0		0
Amortization of restricted stock issued	3,355	3,355	0	0	0	3,355		0	0	0	3,355	0	0	0	0	0	0	3,355	0	3,355
Common stock redeemed for taxes withheld for stock based compensation, net																																								(2,932)	0	0	0	(2,932)	0	0	(2,932)	0	0	0	0
Common stock issued for dividend reinvestment plan																																								288	0	0	0	288	0	0	288	0	0	0	0
Preferred unit distributions		(5,266)							0						0		0	(5,266)	0	(5,266)
Stock issued for stock offerings, net of issuance costs																																								51,608	0	10	0	51,598	0	0	51,608	0	0	0	0
Distributions to partners	(2,483)	(44,149)	0	0	0	0		0	0	0	0	0	0	(2,483)	(2,483)	(2,483)	0	(41,576)	(90)	(41,666)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		48,964							0						0		0	48,964	0	48,964
Preferred stock/unit	(5,266)		0	0	0	0		0		(5,266)	(5,266)	0	0	0		0
Common stock/unit	(41,666)		0	0	0	0		0		(41,576)	(41,576)	0	(90)	0		(90)
Ending Balance at Mar. 31, 2013	$ 1,775,696	$ 1,775,696	$ 325,000	$ 914	$ (15,912)	$ 2,365,607		$ (51,983)	$ (51,983)	$ (860,832)	$ 1,762,794	$ 0	$ (1,193)	$ 14,095	$ 14,095	$ 12,902	$ 0	$ 1,814,777	$ (1,193)	$ 1,761,601