Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Numerator
Net income	$ 97,114	$ 67,000	$ 242,820	$ 169,555
Denominator
Weighted-average shares outstanding, basic	71,738	72,755	72,432	72,727
Effect of dilutive stock awards (in shares)	146	548	248	557
Adjusted weighted-average shares outstanding, diluted	71,884	73,303	72,680	73,284
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.35	$ 0.92	$ 3.35	$ 2.33
Diluted earnings per common share (in dollars per share)	$ 1.35	$ 0.91	$ 3.34	$ 2.31
Anti-dilutive weighted-average shares	57	0	47	38