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	58,603	58,603	0	0	0	0		0	0		57,949	109	545	545	654	57,949	109	58,058
Net Income (Loss) Attributable to Parent	57,949	58,058
Current period other comprehensive income, net	26,453	26,453	0	0	0	0	26,396	26,396	26,396	0	26,396	50	7			0	50	26,446
Other comprehensive income (loss) attributable to noncontrolling interests	57	7
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(1,428)	1,428		0		0	0	0	0		0
Amortization of restricted stock issued	6,978	6,978	0	0	0	6,978		0	0	0	6,978	0	0	0	0	6,978	0	6,978
Common stock redeemed for taxes withheld for stock based compensation, net																									2,921	0	0	0	2,921	0	0	2,921	0	0	0
Common stock issued for dividend reinvestment plan	578	578																								0	0	0	578	0	0	578	0	0	0
Common stock issued for stock offerings, net of issuance costs			0	19	0	98,259		0		0	98,278	0	0		0
Preferred unit distributions		(10,531)							0					0		(10,531)	0	(10,531)
Contributions from partners	39	39	0	0	0	0		0	0	0	0	0	39	39	39	0	0	0
Distributions to partners	(3,311)	(87,356)	0	0	0	0		0	0	0	0	0	(3,311)	(3,311)	(3,311)	(83,874)	(171)	(84,045)
Common units issued as a result of common stock issued by Parent Company, net of repurchases		95,935							0					0		95,935	0	95,935
Cash dividends declared: preferred stock/unit	10,531		0	0	0	0		0		10,531	10,531	0	0		0
Preferred Stock Dividends, Income Statement Impact	10,531
Cash dividends declared - common stock/unit	84,045		0	0	0	0		0		83,874	83,874	171	0		171
Ending Balance at Jun. 30, 2013	1,836,032	1,836,032	325,000	923	(16,352)	2,416,632		(31,319)	(31,319)	(871,266)	1,823,618	(1,165)	13,579	13,579	12,414	1,854,937	(1,165)	1,822,453
Beginning balance at Mar. 31, 2013
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	37,469	37,469
Net Income (Loss) Attributable to Parent	37,129	37,199
Current period other comprehensive income, net	20,706	20,706
Other comprehensive income (loss) attributable to noncontrolling interests	43	4
Preferred Stock Dividends, Income Statement Impact	5,265
Ending Balance at Jun. 30, 2013	1,836,032	1,836,032
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	56,217	56,217	0	0	0	0		0	0		55,403	95	719	719	814	55,403	95	55,498
Net Income (Loss) Attributable to Parent	55,403	55,498
Current period other comprehensive income, net	(19,202)	(19,202)	0	0	0	0	(19,008)	(19,008)	(19,008)	0	(19,008)	(34)	(160)			0	(34)	(19,042)
Other comprehensive income (loss) attributable to noncontrolling interests	(194)	(160)
Adjustments to Additional Paid in Capital, Share-based Compensation, Other Long-term Incentive Plans, Requisite Service Period Recognition	0		0	0	(2,226)	2,226		0		0	0	0	0		0
Amortization of restricted stock issued	5,831	5,831	0	0	0	5,831		0	0	0	5,831	0	0	0	0	5,831	0	5,831
Common stock redeemed for taxes withheld for stock based compensation, net																									3,210	0	0	0	3,210	0	0	3,210	0	0	0
Common stock issued for dividend reinvestment plan	604	604																								0	0	0	604	0	0	604	0	0	0
Preferred unit distributions		(10,531)							0					0		(10,531)	0	(10,531)
Contributions from partners	15,551	15,551	0	0	0	0		0	0	0	0	0	15,551	15,551	15,551	0	0	0
Distributions to partners	(1,008)	(87,692)	0	0	0	0		0	0	0	0	0	(1,008)	(1,008)	(1,008)	(86,532)	(152)	(86,684)
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		(2,606)							0					0		(2,606)	0	(2,606)
Redemption of partnership units	300		0	0	0	0		0		0	0	300	0		300
Cash dividends declared: preferred stock/unit	10,531		0	0	0	0		0			10,531	0	0		0
Preferred Stock Dividends, Income Statement Impact	10,531
Cash dividends declared - common stock/unit	86,684		0	0	0	0		0		86,532	86,532	152	0		152
Ending Balance at Jun. 30, 2014	1,818,402	1,818,402	325,000	923	(18,952)	2,431,928		(36,412)	(36,412)	(916,576)	1,785,911	(1,817)	34,308	34,308	32,491	1,822,323	(1,817)	1,784,094
Beginning balance at Mar. 31, 2014
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	31,161	31,161
Net Income (Loss) Attributable to Parent	30,748	30,801
Current period other comprehensive income, net	(7,921)	(7,921)
Other comprehensive income (loss) attributable to noncontrolling interests	(108)	(92)
Preferred Stock Dividends, Income Statement Impact	5,266
Ending Balance at Jun. 30, 2014	$ 1,818,402	$ 1,818,402