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 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]	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, 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		$ 1,860,758	$ (1,426)	$ 1,841,928
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	109,659	109,659	0	0	0	0		0	0		108,611	185	863	863	1,048		108,611	185	108,796
Net Income (Loss) Attributable to Parent	108,611	108,796
Current period other comprehensive income, net	(16,696)	(16,696)	0	0	0	0	$ (16,696)	(16,559)	(16,559)	0	(16,559)	(29)	(108)				0	(29)	(16,588)
Other comprehensive (loss) income attributable to noncontrolling interests	(137)	(108)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(2,441)	2,441		0		0	0	0	0		0
Restricted stock issued, net of amortization	8,747	8,747	0	0	0	8,747		0	0	0	8,747	0	0	0	0		8,747	0	8,747
Common stock redeemed for taxes withheld for stock based compensation, net																				$ 3,528	$ 0	$ 0	$ 0	$ 3,528	$ 0	$ 0	$ 3,528	$ 0	$ 0	$ 0
Common stock issued for dividend reinvestment plan	895	895																			0	0	0	895	0	0	895	0	0	0
Stock Issued During Period, Value, Conversion of Units																				0	0	0	0	137	0	0	137	(137)	0	(137)
Common stock issued for stock offerings, net of issuance costs			0	9	0	49,291		0		0	49,300	0	0		0
Preferred unit distributions		(15,797)							0					0			(15,797)	0	(15,797)
Contributions from partners	15,933	15,933	0	0	0	0		0	0	0	0	0	15,933	15,933	15,933		0	0	0
Distributions to partners	(5,051)	(135,218)	0	0	0	0		0	0	0	0	0	(5,051)	(5,051)	(5,051)		(129,939)	(228)	(130,167)
Preferred Stock Redemption Premium		(300)							0					0		$ 0	(300)		(300)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		46,667							0					0			46,667	0	46,667
Redemption of partnership units	300		0	0	0	0		0		0	0	300	0		300
Cash dividends declared: preferred stock/unit	15,797		0	0	0	0		0		15,797	15,797	0	0		0
Preferred Stock Dividends, Income Statement Impact	15,797
Cash dividends declared - common stock/unit	130,167		0	0	0	0		0		129,939	129,939	228	0		228
Ending Balance at Sep. 30, 2014	1,874,129	1,874,129	325,000	932	(19,167)	2,484,460		(33,963)	(33,963)	(912,041)	1,845,221	(1,935)	30,843	30,843	28,908		1,879,184	(1,935)	1,843,286
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common units exchanged for common stock of the parent company		0							0					0			(137)	137	0
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	129,004	129,004	0	0	0	0		0	0		127,181	204	1,619	1,619	1,823		127,181	204	127,385
Net Income (Loss) Attributable to Parent	127,181	127,385
Current period other comprehensive income, net	(4,693)	(4,693)	0	0	0	0	$ (4,693)	(4,559)	(4,559)	0	(4,559)	(7)	(127)				0	(7)	(4,566)
Other comprehensive (loss) income attributable to noncontrolling interests	(134)	(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	0	10,441	0	0	0	0		10,441	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
Preferred unit distributions		(15,797)							0					0			(15,797)	0	(15,797)
Contributions from partners	454	454	0	0	0	0		0	0	0	0	0	454	454	454		0	0	0
Distributions to partners	(2,792)	(139,989)	0	0	0	0		0	0	0	0	0	(2,792)	(2,792)	(2,792)		(136,974)	(223)	(137,197)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		(7,858)							0					0			(7,858)	0	(7,858)
Cash dividends declared: preferred stock/unit	15,797		0	0	0	0		0			15,797	0	0		0
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)	$ (62,307)	$ (907,962)	$ 1,879,026	$ (1,940)	$ 30,958	$ 30,958	$ 29,018		$ 1,941,333	$ (1,940)	$ 1,877,086