Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Basic net income per share:
Net income	$ 77,788	$ 90,569	$ 91,616	$ 83,416	$ 93,443	$ 96,725	$ 89,706	$ 77,922	$ 343,389	$ 357,796	$ 305,928
Basic weighted average shares (in shares)									127,991	131,749	134,358
Basic net income per share (in USD per share)	$ 0.61	$ 0.71	$ 0.71	$ 0.65	$ 0.71	$ 0.74	$ 0.68	$ 0.59	$ 2.68	$ 2.72	$ 2.28
Diluted:
Net income	$ 77,788	$ 90,569	$ 91,616	$ 83,416	$ 93,443	$ 96,725	$ 89,706	$ 77,922	$ 343,389	$ 357,796	$ 305,928
Basic weighted average shares (in shares)									127,991	131,749	134,358
Dilutive effect of potential common shares (in shares)									775	1,181	1,183
Diluted weighted average shares (in shares)									128,766	132,930	135,541
Diluted net income per share (in USD per share)	$ 0.61	$ 0.71	$ 0.71	$ 0.64	$ 0.71	$ 0.73	$ 0.67	$ 0.58	$ 2.67	$ 2.69	$ 2.26