Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Numerator
Net income	$ 19,566	$ 30,884	$ 34,591	$ 23,419	$ 23,991	$ 27,657	$ 16,917	$ 7,883	$ 108,460	$ 76,448	$ 72,481
Denominator
Weighted-average shares outstanding, basic									72,386	53,241	26,184
Effect of dilutive stock awards									205	274	506
Adjusted weighted-average shares outstanding, diluted									72,591	53,515	26,690
Net Income per Share
Basic earnings per common share	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 0.33	$ 0.38	$ 0.41	$ 0.30	$ 1.50	$ 1.44	$ 2.77
Diluted earnings per common share	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 0.33	$ 0.38	$ 0.41	$ 0.30	$ 1.49	$ 1.43	$ 2.72
Antidilutive awards excluded from computation of earnings per common share (in shares)									88	70