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]	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] 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	$ 1,788,480	$ (1,153)	$ 1,729,612
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	99,268	99,268	0	0	0	0		0	0		98,213	183	872	872	1,055	98,213	183	98,396
Net Income (Loss) Attributable to Parent	98,213	98,396
Current period other comprehensive income, net	29,348	29,348	0	0	0	0	29,285	29,285	29,285	0	29,285	55	8			0	55	29,340
Other comprehensive income (loss) attributable to noncontrolling interests	63	8
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(1,616)	1,616		0		0	0	0	0		0
Amortization of restricted stock issued	10,600	10,600	0	0	0	10,600		0	0	0	10,600	0	0	0	0	10,600	0	10,600
Common stock redeemed for taxes withheld for stock based compensation, net																									2,927	0	0	0	2,927	0	0	2,927	0	0	0
Common stock issued for dividend reinvestment plan	831	831																								0	0	0	831	0	0	831	0	0	0
Stock Issued During Period, Value, Conversion of Units	302	302																							0	0	0	0	302	0	0	302	(302)	0	(302)
Common stock issued for stock offerings, net of issuance costs			0	19	0	99,734		0		0	99,753	0	0		0
Preferred unit distributions		(15,797)							0					0		(15,797)	0	(15,797)
Contributions from partners	347	347	0	0	0	0		0	0	0	0	0	347	347	347	0	0	0
Distributions to partners	(3,635)	(130,239)	0	0	0	0		0	0	0	0	0	(3,635)	(3,635)	(3,635)	(126,359)	(245)	(126,604)
Preferred Stock Redemption Premium		0							0					0		0	0	0
Common units issued as a result of common stock issued by Parent Company, net of repurchases		97,657							0					0		97,657	0	97,657
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	126,604		0	0	0	0		0		126,359	126,359	245	0		245
Common units exchanged for common stock of the parent company		0							0					0		302	(302)	0
Ending Balance at Sep. 30, 2013	1,837,095	1,837,095	325,000	923	(16,540)	2,422,466		(28,430)	(28,430)	(878,753)	1,824,666	(1,462)	13,891	13,891	12,429	1,853,096	(1,462)	1,823,204
Beginning balance at Jun. 30, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	40,664	40,664
Net Income (Loss) Attributable to Parent	40,264	40,337
Current period other comprehensive income, net	2,896	2,896
Other comprehensive income (loss) attributable to noncontrolling interests	6	1
Preferred Stock Dividends, Income Statement Impact	5,266
Ending Balance at Sep. 30, 2013	1,837,095	1,837,095
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	(16,559)	(16,559)	(16,559)	0	(16,559)	(29)	(108)			0	(29)	(16,588)
Other comprehensive income (loss) 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
Amortization of restricted stock issued	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	137	137																							0	0	0	0	137	0	0	137	(137)	0	(137)
Common stock issued for stock offerings, net of issuance costs																									(49,300)	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	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
Common units exchanged for common stock of the parent company		0							0					0		137	(137)	0
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
Beginning balance at Jun. 30, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	53,440	53,440
Net Income (Loss) Attributable to Parent	53,208	53,298
Current period other comprehensive income, net	2,504	2,504
Other comprehensive income (loss) attributable to noncontrolling interests	56	52
Preferred Stock Dividends, Income Statement Impact	5,266
Ending Balance at Sep. 30, 2014	$ 1,874,129	$ 1,874,129