Background (Details) (USD $)	0 Months Ended			0 Months Ended					0 Months Ended
	Feb. 07, 2011	Dec. 31, 2012	Dec. 31, 2011	Feb. 07, 2011 IPO [Member]	Feb. 01, 2011 IPO [Member]	Feb. 01, 2011 Corporate [Member] IPO [Member]	Feb. 01, 2011 Investor [Member] IPO [Member]	Feb. 03, 2011 Overallotment Option [Member] IPO [Member]	Feb. 07, 2011 Common Class B [Member] IPO [Member]
Organization, Consolidation and Presentation of Financial Statements [Line Items]
Common stock, shares authorized (in shares)		100,000,000	100,000,000	100,000,000	5,360,000			804,000
Sale of stock, price per share (in dollars per share)					$ 16.00			$ 16.00
Common Stock, Value, Issued		$ 25,000	$ 24,000		$ 85,800,000	$ 57,200,000	$ 28,600,000	$ 12,900,000
Common stock, shares issued (in shares)		24,902,000	24,370,000			3,574,285	1,785,715
Proceeds from Issuance Initial Public Offering				64,200,000
Common Stock Underwriting and Commissions Expense				4,900,000
Other Expenses				3,000,000
Dividends, Common Stock									$ 29,600,000
Stock Issued During Period, Shares, Conversion of Convertible Securities (in shares)				11,089,201
Preferred stock, shares authorized (in shares)		10,000,000	10,000,000	10,000,000
Common stock, par value (in dollars per share)		$ 0.001	$ 0.001	$ 0.001
Preferred stock, par value (in dollars per share)		$ 0.001	$ 0.001
Stock Issued During Period, Shares, Reverse Stock Splits	0.786