Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Numerator
Net income	$ 78,143	$ 73,084	$ 110,078	$ 135,004
Denominator
Weighted-average shares outstanding, basic (in shares)	69,370	70,770	69,359	71,173
Effect of dilutive stock awards (in shares)	191	143	217	174
Adjusted weighted-average shares outstanding, diluted (in shares)	69,561	70,913	69,576	71,347
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.13	$ 1.03	$ 1.59	$ 1.90
Diluted earnings per common share (in dollars per share)	$ 1.12	$ 1.03	$ 1.58	$ 1.89
Anti-dilutive weighted-average shares (in shares)	17	54	52	69