Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Numerator
Net income	$ 31,935	$ 61,920
Denominator
Weighted-average shares outstanding, basic (in shares)	69,348	71,572
Effect of dilutive stock awards (in shares)	244	204
Adjusted weighted-average shares outstanding, diluted (in shares)	69,592	71,777
Net income per share
Basic earnings per common share (in dollars per share)	$ 0.46	$ 0.87
Diluted earnings per common share (in dollars per share)	$ 0.46	$ 0.86
Anti-dilutive weighted-average shares (in shares)	88	85