Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator
Net income	$ 76,704	$ 64,849	$ 145,706	$ 102,555
Denominator
Weighted-average shares outstanding, basic	72,518	72,740	72,784	72,712
Effect of dilutive stock awards (in shares)	283	554	299	562
Adjusted weighted-average shares outstanding, diluted	72,801	73,294	73,083	73,274
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.06	$ 0.89	$ 2.00	$ 1.41
Diluted earnings per common share (in dollars per share)	$ 1.05	$ 0.88	$ 1.99	$ 1.40
Anti-dilutive weighted-average shares	56	57	42	56