Stockholders' Equity (Details) (USD $)	1 Months Ended				9 Months Ended
	Aug. 31, 2011 Y	May 31, 2011	Dec. 31, 2010	Aug. 31, 2009	Sep. 30, 2011	Aug. 17, 2009
Stockholders' Equity
Authorized capital stock, preferred stock (in shares)			100,000,000		100,000,000
Par value per share, preferred stock (in dollars per share)			$ 0.01		$ 0.01
Authorized capital stock, common stock (in shares)			500,000,000		500,000,000
Par value per share, common stock (in dollars per share)			$ 0.01		$ 0.01
Proceeds from public offering 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
Dividend declared on common stock (in dollars per share)					$ 0.44
Authorized amount of shares repurchased	$ 100,000,000
Period to repurchase shares (in years)	1
Shares repurchased (in shares)					342,000
Aggregate cost of repurchased shares					$ 5,981,000
Weighted-average share cost of repurchased shares (in dollars per share)					$ 17.49