Consolidated Statement of Changes in Equity - USD ($) shares in Thousands, $ in Thousands	Total	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]	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 Interest [Member] Parent Company [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] Noncontrolling Interest [Member] Parent Company [Member]	Noncontrolling Interest [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Limited Partner [Member]	General Partner [Member]
Beginning balance at Dec. 31, 2014	$ 1,936,482	$ 1,936,482		$ 325,000	$ 941	$ (19,382)	$ 2,540,153		$ (57,748)	$ (882,372)	$ 1,906,592	$ (1,914)	$ 31,804	$ 29,890												$ 31,804	$ (57,748)	$ (1,914)	$ 1,964,340
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	129,004	129,004	$ 129,004	0	0	0	0		0		127,181	204	1,619	1,823												1,619	0	204	127,181
Net Income (Loss) Attributable to Parent		127,181	127,385
Current period other comprehensive income, net	(4,693)	(4,693)	(4,693)	0	0	0	0	$ (4,559)	(4,559)	0	(4,559)	(7)	(127)														(4,559)	(7)	0
Other comprehensive income (loss) attributable to noncontrolling interests		(134)	(127)																							(127)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition		0		0	0	(56)	56		0	0	0	0	0	0
Restricted stock issued, net of amortization	10,441	10,441		0	0	0	10,441		0	0	10,441	0	0	0												0	0	0	10,441
Common stock redeemed for taxes withheld for stock based compensation, net															$ 9,770	$ 0	$ 0	$ 0	$ 9,770	$ 0	$ 0	$ 9,770	$ 0	$ 0	$ 0
Common stock issued for dividend reinvestment plan		966	966													0	0	0	966	0	0	966	0	0	0
Common stock issued for stock offerings, net of issuance costs		946		0	1	0	945		0	0	946	0	0	0
Contributions from partners	454	454		0	0	0	0		0	0	0	0	454	454												454	0	0	0
Distributions to partners	(139,989)	(2,792)		0	0	0	0		0	0	0	0	(2,792)	(2,792)												(2,792)	0	(223)	(136,974)
Cash dividends declared: preferred stock/unit	15,797	15,797		0	0	0	0		0	15,797	15,797	0	0	0												0	0	0	15,797
Preferred Stock Dividends, Income Statement Impact		(15,797)
Cash dividends declared - common stock/unit		137,197		0	0	0	0		0	136,974	136,974	223	0	223
Ending Balance at Sep. 30, 2015	$ 1,908,044	1,908,044		325,000	942	(19,438)	2,542,791		(62,307)	(907,962)	1,879,026	(1,940)	30,958	29,018												$ 30,958	$ (62,307)	$ (1,940)	$ 1,941,333
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Partners' Capital Account, Units, Redeemed	7,858																									0	0	0	7,858
Beginning balance at Dec. 31, 2015	$ 2,082,620	2,082,620		325,000	972	(19,658)	2,742,508		(58,693)	(936,020)	2,054,109	(1,975)	30,486	28,511												$ 30,486	$ (58,693)	$ (1,975)	$ 2,112,802
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	105,334	105,334	105,334	0	0	0	0		0		103,789	165	1,380	1,545												1,380	0	165	103,789
Net Income (Loss) Attributable to Parent		103,789	103,954
Current period other comprehensive income, net	22,815	22,815	22,815	0	0	0	0	$ 22,954	22,954	0	22,954	33	(172)														22,954	33	0
Other comprehensive income (loss) attributable to noncontrolling interests		(139)	(172)																							(172)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition		0		0	0	2,776	(2,776)		0	0	0	0	0	0
Restricted stock issued, net of amortization	9,967	9,967		0	2	0	9,965		0	0	9,967	0	0	0												0	0	0	9,967
Common stock redeemed for taxes withheld for stock based compensation, net															7,835	0	0	0	7,835	0	0	7,835	0	0	0
Common stock issued for dividend reinvestment plan		804	$ 804													0	0	0	804	0	0	804	0	0	0
Common stock issued for stock offerings, net of issuance costs															$ 549,545	$ 0	$ 71	$ 0	$ 549,474	$ 0	$ 0	$ 549,545	$ 0	$ 0	$ 0
Contributions from partners	8,675	8,675		0	0	0	0		0	0	0	0	8,675	8,675												8,675	0	0	0
Distributions to partners	(155,844)	(5,762)		0	0	0	(538)		0	0	(538)	0	(5,224)	(5,224)												(5,224)	0	(229)	(150,391)
Common units redeemed as a result of common stock redeemed by Parent Company, net of issuances	(542,514)																									0	0	0	(542,514)
Cash dividends declared: preferred stock/unit	15,797	15,797		0	0	0	0		0		15,797	0	0	0												0	0	0	15,797
Preferred Stock Dividends, Income Statement Impact		(15,797)
Cash dividends declared - common stock/unit		150,082		0	0	0	0		0	149,853	149,853	229	0	229
Ending Balance at Sep. 30, 2016	$ 2,600,284	$ 2,600,284		$ 325,000	$ 1,045	$ (16,882)	$ 3,291,602		$ (35,739)	$ (997,881)	$ 2,567,145	$ (2,006)	$ 35,145	$ 33,139												$ 35,145	$ (35,739)	$ (2,006)	$ 2,602,884