Stockholders' Equity (Details) (USD $)	1 Months Ended				5 Months Ended	12 Months Ended
	Aug. 31, 2011 Y	May 31, 2011	Dec. 31, 2010	Aug. 31, 2009	Dec. 31, 2011	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Aug. 17, 2009
Stockholders' Equity
Authorized capital stock, preferred stock (in shares)			100,000,000		100,000,000	100,000,000
Par value per share, preferred stock (in dollars per share)			$ 0.01		$ 0.01	$ 0.01
Authorized capital stock, common stock (in shares)			500,000,000		500,000,000	500,000,000
Par value per share, common stock (in dollars per share)			$ 0.01		$ 0.01	$ 0.01
Issuance of common stock (in shares)		22,000,000	23,000,000	47,575,000
Issuance of common stock to related party (in shares)				1,000,000
Stock issuance price (in dollars per share)		$ 21.67	$ 19.73							$ 20
Loss contingencies
Deferred liability recorded and offsetting reduction in additional paid-in capital			$ 27,195,000
Authorized amount of shares repurchased	100,000,000
Period to repurchase common stock (in years)	1
Shares repurchased					625,850
Aggregate cost of repurchased shares						10,642,000
Weighted-average share cost of repurchased shares (in dollars per share)						$ 17.00
Dividend declared on common stock (in dollars per share)					$ 0.44	$ 0.44	$ 0.44	$ 0.44	$ 0.42
Underwriting for initial public offering
Loss contingencies
Amount payable if contingency occurs					18,100,000	18,100,000
Number of prior quarters for calculation of threshold Core Earnings percentage (in quarters)						4
Number of quarters which act as threshold to trigger contingency (in quarters)						24
Threshold percentage of Core Earnings for payment of underwriters' fees (as a percent)						8.00%
Underwriting fee paid by manager						9,100,000
Amount paid due to occurrence of contingency						27,200,000
Deferred liability recorded and offsetting reduction in additional paid-in capital					$ 27,200,000	$ 27,200,000