Net Income per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator:
Net income	$ 43,193	$ 61,103	$ 42,068	$ 30,554	$ 19,566	$ 30,884	$ 34,591	$ 23,419	$ 176,918	$ 108,460	$ 76,448
Denominator:
Weighted-average shares outstanding, basic									72,593	72,386	53,241
Effect of dilutive stock awards									406	205	274
Adjusted weighted-average shares outstanding, diluted									72,999	72,591	53,515
Net Income per Share:
Basic earnings per common share (in dollars per share)	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 2.44	$ 1.50	$ 1.44
Diluted earnings per common share (in dollars per share)	$ 0.59	$ 0.84	$ 0.58	$ 0.42	$ 0.27	$ 0.43	$ 0.48	$ 0.32	$ 2.42	$ 1.49	$ 1.43
Antidilutive awards excluded from computation of earnings per common share (in shares)									1	88	70