Earnings per Share (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Numerator
Net income	$ 64,849	$ 42,068	$ 102,555	$ 72,622
Denominator
Weighted-average shares outstanding, basic	72,740	72,593	72,712	72,540
Effect of dilutive stock awards (in shares)	554	399	562	358
Adjusted weighted-average shares outstanding, diluted	73,294	72,992	73,274	72,898
Net Income per Share
Basic earnings per common share (in dollars per share)	$ 0.89	$ 0.58	$ 1.41	$ 1.00
Diluted earnings per common share (in dollars per share)	$ 0.88	$ 0.58	$ 1.40	$ 1.00
Anti-dilutive weighted-average shares	57	2	56	1