Net Income Per Share - Calculation of Net Income 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
Earnings Per Share [Abstract]
Net income	$ 84,700	$ 90,569	$ 243,537	$ 265,601
Basic:
Weighted average shares (in shares)	123,569	127,615	124,567	128,491
Diluted:
Weighted average shares (in shares)	123,569	127,615	124,567	128,491
Dilutive effect of potential common shares (in shares)	631	576	666	725
Diluted weighted average shares (in shares)	124,200	128,191	125,233	129,216
Net income per share:
Basic (in usd per share)	$ 0.69	$ 0.71	$ 1.96	$ 2.07
Diluted (in usd per share)	$ 0.68	$ 0.71	$ 1.94	$ 2.06