Consolidated Statement of Changes in Equity - USD ($) shares in Thousands, $ 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 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]	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] 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] 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] 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, 2014	$ 1,936,482	$ 1,936,482	$ 325,000	$ 941	$ (19,382)	$ 2,540,153		$ (57,748)	$ (57,748)	$ (882,372)	$ 1,906,592	$ (1,914)	$ 31,804	$ 31,804	$ 29,890	$ 1,964,340	$ (1,914)	$ 1,904,678
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	30,992	30,992	0	0	0	0		0	0		30,440	49	503	503	552	30,440	49	30,489
Net Income (Loss) Attributable to Parent	30,440	30,489
Current period other comprehensive income, net	(11,633)	(11,633)	0	0	0	0	$ (11,529)	(11,529)	(11,529)	0	(11,529)	(18)	(86)			0	(18)	(11,547)
Other comprehensive loss attributable to noncontrolling interests	(104)	(86)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(1,256)	1,256		0		0	0	0	0		0
Restricted stock issued, net of amortization	3,548	3,548	0	2	0	3,546		0	0	0	3,548	0	0	0	0	3,548	0	3,548
Common stock redeemed for taxes withheld for stock based compensation, net																								$ 9,850	$ 0	$ 0	$ 0	$ 9,850	$ 0	$ 0	$ 9,850	$ 0	$ 0	$ 0
Common stock issued for dividend reinvestment plan	374	374																							0	0	0	374	0	0	374	0	0	0
Common stock issued for stock offerings, net of issuance costs			0	0	0	992		0		0	992	0	0		0
Contributions from partners	13	13	0	0	0	0		0	0	0	0	0	13	13	13	0	0	0
Distributions to partners	(1,050)	(46,773)	0	0	0	0		0	0	0	0	0	(1,050)	(1,050)	(1,050)	(45,647)	(76)	(45,723)
Cash dividends declared: preferred stock/unit	5,266	5,266	0	0	0	0		0		5,266	5,266	0	0		0				$ 0	$ 0	$ 5,266	$ 0	$ 5,266
Preferred Stock Dividends, Income Statement Impact	5,266
Cash dividends declared - common stock/unit	45,723		0	0	0	0		0		45,647	45,647	76	0		76
Ending Balance at Mar. 31, 2015	1,898,879	$ 1,898,879	325,000	943	(20,638)	2,536,471		(69,277)	$ (69,277)	(902,845)	1,869,654	(1,959)	31,184	$ 31,184	29,225	$ 1,938,931	$ (1,959)	$ 1,867,695
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Partners' Capital Account, Units, Redeemed		8,484							0					0		8,484	0	8,484
Beginning balance at Dec. 31, 2015	2,082,620	$ 2,082,620							$ (58,693)					$ 30,486		$ 2,112,802	$ (1,975)	$ 2,052,134
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,577	53,577	0	0	0	0		0	0		53,143	85	349	349	434	53,143	85	53,228
Net Income (Loss) Attributable to Parent	53,143	53,228
Current period other comprehensive income, net	(14,368)	(14,368)	0	0	0	0	$ (14,200)	(14,200)	(14,200)	0	(14,200)	(22)	(146)			0	(22)	(14,222)
Other comprehensive loss attributable to noncontrolling interests	(168)	(146)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	1,287	(1,287)		0		0	0	0	0		0
Restricted stock issued, net of amortization	3,402	3,402	0	2	0	3,400		0	0	0	3,402	0	0	0	0	3,402	0	3,402
Common stock redeemed for taxes withheld for stock based compensation, net																								7,950	0	0	0	7,950	0	0	7,950	0	0	0
Common stock issued for dividend reinvestment plan	292	292																							0	0	0	292	0	0	292	0	0	0
Common stock issued for stock offerings, net of issuance costs																								$ 12,293	$ 0	$ 2	$ 0	$ 12,291	$ 0	$ 0	$ 12,293	$ 0	$ 0	$ 0
Contributions from partners	8,389	8,389	0	0	0	0		0	0	0	0	0	8,389	8,389	8,389	0	0	0
Distributions to partners	(1,737)	(50,616)	0	0	0	(350)		0	0	0	(350)	0	(1,387)	(1,387)	(1,387)	49,152	(77)	(49,229)
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances		4,635							0					0		4,635	0	4,635
Cash dividends declared: preferred stock/unit	5,266	5,266	0	0	0	0		0			5,266	0	0		0				$ 0	$ 0	$ 5,266	$ 0	$ 5,266
Preferred Stock Dividends, Income Statement Impact	5,266
Cash dividends declared - common stock/unit	48,879		0	0	0	0		0		48,802	48,802	77	0		77
Ending Balance at Mar. 31, 2016	$ 2,082,373	$ 2,082,373	$ 325,000	$ 976	$ (18,371)	$ 2,748,904		$ (72,893)	$ (72,893)	$ (936,945)	$ 2,046,671	$ (1,989)	$ 37,691	$ 37,691	$ 35,702	$ 2,119,564	$ (1,989)	$ 2,044,682