Consolidated Statements of Equity (Unaudited) (USD $) In Thousands, except Share data, unless otherwise specified	Total	Total Stockholders' Equity	Preferred Stock [Member]	Common Stock [Member]	Common Stock Additional Paid-in Capital	Common Stock Distributions in Excess of Earnings	Noncontrolling Interests - Common Units of the Operating Partnership	Series A Preferred Units [Member]	Series A Preferred Units [Member] Total Stockholders' Equity	Series A Preferred Units [Member] Common Stock Distributions in Excess of Earnings
Beginning Balance at Dec. 31, 2011	$ 1,327,482	$ 1,293,717	$ 121,582	$ 588	$ 1,448,997	$ (277,450)	$ 33,765
Beginning Balance, shares at Dec. 31, 2011				58,819,717
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	83,471	81,763				81,763	1,708
Issuance of Series G and Series H Preferred stock	192,411	192,411	192,411
Redemption of Series E and Series F Preferred stock, shares			(121,582)
Redemption of Preferred stock and units	(126,500)	(126,500)				(4,918)		(2,062)	(2,062)	(2,062)
Issuance of common stock, shares				15,813,189
Issuance of common stock	662,371	662,371		159	662,212
Issuance of share-based compensation awards, shares				62,137
Issuance of share-based compensation awards	957	957			957
Noncash amortization of share-based compensation	6,182	6,182			6,182
Repurchase of common stock and restricted stock units, shares				(22,312)
Repurchase of common stock and restricted stock units	(603)	(603)			(603)
Settlement of restricted stock units for common stock, shares				5,208
Settlement of restricted stock units for common stock value	(133)	(133)			(133)
Exercise of stock options, net, units				5,000
Exercise of stock options, net	129	129			129
Issuance of common units	5,604						5,604
Exchange of common units of the Operating Parternship, shares				10,000
Exchange of common units of the Operating Partnership	0	231			231		(231)
Adjustment for noncontrolling interest	0	(3,198)			(3,198)		3,198
Preferred dividends and distributions	(10,795)	(10,795)				(10,795)
Dividends declared per common share and common unit ($1.05 per share/unit)	(77,146)	(75,303)				(75,303)	(1,843)
Ending Balance at Sep. 30, 2012	2,061,368	2,019,167	192,411	747	2,114,774	(288,765)	42,201
Ending Balance, shares at Sep. 30, 2012				74,692,939
Beginning Balance at Dec. 31, 2012	2,235,933	2,189,630	192,411	749	2,126,005	(129,535)	46,303
Beginning Balance, shares at Dec. 31, 2012				74,926,981
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	21,518	21,252				21,252	266
Issuance of common stock, shares				7,215,838
Issuance of common stock	349,951	349,951		72	349,879
Issuance of share-based compensation awards, shares				0
Issuance of share-based compensation awards	1,075	1,075			1,075
Noncash amortization of share-based compensation	7,096	7,096			7,096
Repurchase of common stock and restricted stock units, shares				(34,164)
Repurchase of common stock and restricted stock units	(1,803)	(1,803)			(1,803)
Settlement of restricted stock units for common stock, shares				4,363
Settlement of restricted stock units for common stock value	(10)	(10)			(10)
Exercise of stock options, net, units				473
Exercise of stock options, net	128	128			128
Issuance of common units	4,885						4,885
Adjustment for noncontrolling interest	0	(5,946)			(5,946)		5,946
Preferred dividends and distributions	(9,938)	(9,938)				(9,938)
Dividends declared per common share and common unit ($1.05 per share/unit)	(84,741)	(82,827)				(82,827)	(1,914)
Ending Balance at Sep. 30, 2013	$ 2,524,094	$ 2,468,608	$ 192,411	$ 821	$ 2,476,424	$ (201,048)	$ 55,486
Ending Balance, shares at Sep. 30, 2013				82,113,491