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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share [Abstract]
Net income	$ 90,569	$ 96,725	$ 265,601	$ 264,353
Basic:
Weighted average shares (in shares)	127,615	131,285	128,491	132,280
Diluted:
Weighted average shares (in shares)	127,615	131,285	128,491	132,280
Dilutive effect of potential common shares (in shares)	576	1,203	725	1,156
Diluted weighted average shares (in shares)	128,191	132,488	129,216	133,436
Net income per share:
Basic (in usd per share)	$ 0.71	$ 0.74	$ 2.07	$ 2.00
Diluted (in usd per share)	$ 0.71	$ 0.73	$ 2.06	$ 1.98