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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share [Abstract]
Net income	$ 113,564	$ 115,242	$ 109,315	$ 96,167	$ 47,047	$ 84,700	$ 80,316	$ 78,521	$ 434,288	$ 290,584	$ 343,389
Basic:
Weighted average shares (in shares)									120,513	124,152	127,991
Diluted:
Weighted average shares (in shares)									120,513	124,152	127,991
Dilutive effect of potential common shares (in shares)									1,089	740	775
Diluted weighted average shares (in shares)									121,602	124,892	128,766
Net income per share:
Basic (usd per share)	$ 0.96	$ 0.96	$ 0.90	$ 0.79	$ 0.38	$ 0.69	$ 0.64	$ 0.63	$ 3.60	$ 2.34	$ 2.68
Diluted (usd per share)	$ 0.95	$ 0.95	$ 0.89	$ 0.78	$ 0.38	$ 0.68	$ 0.64	$ 0.62	$ 3.57	$ 2.33	$ 2.67