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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 47,047	$ 84,700	$ 80,316	$ 78,521	$ 77,788	$ 90,569	$ 91,616	$ 83,416	$ 290,584	$ 343,389	$ 357,796
Basic:
Weighted average shares (in shares)									124,152	127,991	131,749
Diluted:
Weighted average shares (in shares)									124,152	127,991	131,749
Dilutive effect of potential common shares (in shares)									740	775	1,181
Diluted weighted average shares (in shares)									124,892	128,766	132,930
Net income per share:
Basic (usd per share)	$ 0.38	$ 0.69	$ 0.64	$ 0.63	$ 0.61	$ 0.71	$ 0.71	$ 0.65	$ 2.34	$ 2.68	$ 2.72
Diluted (usd per share)	$ 0.38	$ 0.68	$ 0.64	$ 0.62	$ 0.61	$ 0.71	$ 0.71	$ 0.64	$ 2.33	$ 2.67	$ 2.69