Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Numerator
Net income	$ 81,382	$ 97,114	$ 216,386	$ 242,820
Denominator
Weighted-average shares outstanding, basic (in shares)	69,727	71,738	70,689	72,432
Effect of dilutive stock awards (in shares)	81	146	143	248
Adjusted weighted-average shares outstanding, diluted (in shares)	69,808	71,884	70,832	72,680
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.17	$ 1.35	$ 3.06	$ 3.35
Diluted earnings per common share (in dollars per share)	$ 1.17	$ 1.35	$ 3.05	$ 3.34
Anti-dilutive weighted-average shares	122	57	87	47