Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator
Net income	$ 60,190	$ 81,382	$ 170,268	$ 216,386
Denominator
Weighted-average shares outstanding, basic (in shares)	69,370	69,727	69,363	70,689
Effect of dilutive stock awards (in shares)	88	81	174	143
Adjusted weighted-average shares outstanding, diluted (in shares)	69,458	69,808	69,537	70,832
Net income per share
Basic earnings per common share (in dollars per share)	$ 0.87	$ 1.17	$ 2.45	$ 3.06
Diluted earnings per common share (in dollars per share)	$ 0.87	$ 1.17	$ 2.45	$ 3.05
Anti-dilutive weighted-average shares (in shares)	124	122	76	87