Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Numerator
Net income	$ 73,084	$ 76,704	$ 135,004	$ 145,706
Denominator
Weighted-average shares outstanding, basic	70,770	72,518	71,173	72,784
Effect of dilutive stock awards (in shares)	143	283	174	299
Adjusted weighted-average shares outstanding, diluted	70,913	72,801	71,347	73,083
Net income per share
Basic earnings per common share (in dollars per share)	$ 1.03	$ 1.06	$ 1.90	$ 2.00
Diluted earnings per common share (in dollars per share)	$ 1.03	$ 1.05	$ 1.89	$ 1.99
Anti-dilutive weighted-average shares	54	56	69	42