Consolidated Statement of Changes in Equity (USD $) In Thousands, unless otherwise specified	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 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]	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 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, 2012	$ 1,745,911	$ 1,745,911	$ 325,000	$ 904	$ (14,924)	$ 2,312,310		$ (57,715)	$ (57,715)	$ (834,810)	$ 1,730,765	$ (1,153)	$ 16,299	$ 16,299	$ 15,146	$ 0	$ 1,788,480	$ (1,153)	$ 1,729,612
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,134	21,134	0	0	0	0		0	0	20,820	20,820	39	275	275	314	0	20,820	39	20,859
Current period other comprehensive income, net	5,747	5,747	0	0	0	0	5,732	5,732	5,732	0	5,732	11	4	4	15	0	0	11	5,743
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(988)	988		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	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
Common stock issued for dividend reinvestment plan	288	288																										0	0	0	288	0	0	288	0	0	0
Common stock issued for stock offerings, net of issuance costs	51,608		0	10	0	51,598		0		0	51,608	0	0		0
Preferred unit distributions		(5,266)							0					0		0	(5,266)	0	(5,266)
Distributions to partners	(2,483)	(44,149)	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
Cash dividends declared: preferred stock/unit	5,266		0	0	0	0		0		5,266	5,266	0	0		0
Cash dividends declared - common stock/unit	41,666		0	0	0	0		0		41,576	41,576	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	(1,193)	14,095	14,095	12,902	0	1,814,777	(1,193)	1,761,601
Beginning balance at Dec. 31, 2013	1,861,134	1,861,134	325,000	923	(16,726)	2,426,477		(17,404)	(17,404)	(874,916)	1,843,354	(1,426)	19,206	19,206	17,780	0	1,860,758	(1,426)	1,841,928
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	25,056	25,056	0	0	0	0		0	0	24,655	24,655	42	359	359	401	0	24,655	42	24,697
Current period other comprehensive income, net	(11,281)	(11,281)	0	0	0	0	(11,195)	(11,195)	(11,195)	0	(11,195)	(20)	(66)	(66)	(86)	0	0	(20)	(11,215)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(1,625)	1,625		0		0	0	0	0		0
Amortization of restricted stock issued	2,915	2,915	0	0	0	2,915		0	0	0	2,915	0	0	0	0	0	2,915	0	2,915
Common stock redeemed for taxes withheld for stock based compensation, net																											3,164	0	0	0	3,164	0	0	3,164	0	0	0
Common stock issued for dividend reinvestment plan	313	313																										0	0	0	313	0	0	313	0	0	0
Preferred unit distributions		(5,266)							0					0		0	(5,266)	0	(5,266)
Contributions from partners	15,443	15,443	0	0	0	0		0	0	0	0	0	15,443	15,443	15,443	0	0	0	0
Distributions to partners	(426)	(43,764)	0	0	0	0		0	0	0	0	0	(426)	(426)	(426)	0	(43,260)	(78)	(43,338)
Preferred Stock Redemption Premium																				(300)	0	0	0	0	(300)	(300)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		(2,851)							0					0		0	(2,851)	0	(2,851)
Redemption of preferred units	300		0	0	0	0		0		0	0	300	0		300
Cash dividends declared: preferred stock/unit	5,266		0	0	0	0		0			5,266	0	0		0
Cash dividends declared - common stock/unit	43,338		0	0	0	0		0		43,260	43,260	78	0		78
Ending Balance at Mar. 31, 2014	$ 1,841,086	$ 1,841,086	$ 325,000	$ 923	$ (18,351)	$ 2,428,166		$ (28,599)	$ (28,599)	$ (898,787)	$ 1,808,352	$ (1,782)	$ 34,516	$ 34,516	$ 32,734	$ 0	$ 1,836,951	$ (1,782)	$ 1,806,570