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
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	78,671	76,876				76,876	1,795
Issuance of Series G Preferred stock	96,155	96,155	96,155
Redemption of Series E and Series F Preferred stock, shares			(121,582)
Redemption of Series E and Series F Preferred stock	(126,500)	(126,500)				(4,918)
Issuance of common stock, shares (Note 7)				9,487,500
Issuance of common stock (Note 7)	382,063	382,063		95	381,968
Issuance of share-based compensation awards, shares (Note 9)				59,938
Issuance of share-based compensation awards (Note 9)	294	294		0	294
Noncash amortization of share-based compensation (Note 9)	1,469	1,469			1,469
Repurchase of common stock and restricted stock units, shares (Note 9)				(22,312)
Repurchase of common stock and restricted stock units (Note 9)	(603)	(603)			(603)
Exercise of stock options, shares				5,000
Exercise of stock options	129	129			129
Adjustment for noncontrolling interest	0	(4,578)			(4,578)		4,578
Preferred dividends and distributions	(4,418)	(4,418)				(4,418)
Dividends declared per common share and common unit ($0.35 per share/ unit)	(24,892)	(24,289)				(24,289)	(603)
Ending Balance at Mar. 31, 2012	1,729,850	1,690,315	96,155	683	1,827,676	(234,199)	39,535
Ending Balance, shares at Mar. 31, 2012				68,349,843
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			74,926,981
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	2,388	2,410				2,410	(22)
Issuance of common stock, shares (Note 7)				453,679
Issuance of common stock (Note 7)	23,395	23,395		4	23,391
Issuance of share-based compensation awards, shares (Note 9)				0
Issuance of share-based compensation awards (Note 9)	336	336			336
Noncash amortization of share-based compensation (Note 9)	2,422	2,422			2,422
Repurchase of common stock and restricted stock units, shares (Note 9)				(33,534)
Repurchase of common stock and restricted stock units (Note 9)	(1,199)	(1,199)			(1,199)
Settlement of restricted stock units for common stock, shares (Note 9)				2,579
Settlement of restricted stock units for common stock value (Note 9)	(10)	(10)			(10)
Exercise of stock options, shares	0
Adjustment for noncontrolling interest	0	(1,893)			(1,893)		1,893
Preferred dividends and distributions	(3,313)	(3,313)				(3,313)
Dividends declared per common share and common unit ($0.35 per share/ unit)	(27,412)	(26,773)				(26,773)	(639)
Ending Balance at Mar. 31, 2013	$ 2,232,540	$ 2,185,005	$ 192,411	$ 753	$ 2,149,052	$ (157,211)	$ 47,535
Ending Balance, shares at Mar. 31, 2013	75,349,705			75,349,705