Net Income Per Share - Calculation of Net Income 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
Earnings Per Share [Abstract]
Net income	$ 91,616	$ 89,706	$ 175,032	$ 167,628
Basic:
Weighted average shares (in shares)	128,586	132,499	128,933	132,786
Diluted:
Weighted average shares (in shares)	128,586	132,499	128,933	132,786
Dilutive effect of potential common shares (in shares)	743	1,054	800	1,132
Diluted weighted average shares (in shares)	129,329	133,553	129,733	133,918
Net income per share:
Basic (in usd per share)	$ 0.71	$ 0.68	$ 1.36	$ 1.26
Diluted (in usd per share)	$ 0.71	$ 0.67	$ 1.35	$ 1.25