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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income	$ 112,842	$ 117,181	$ 114,612	$ 109,798	$ 113,564	$ 115,242	$ 109,315	$ 96,167	$ 454,433	$ 434,288	$ 290,584
Basic:
Weighted average shares (in shares)									115,656	120,513	124,152
Diluted:
Weighted average shares (in shares)									115,656	120,513	124,152
Dilutive effect of potential common shares (in shares)									755	1,089	740
Diluted weighted average shares (in shares)									116,411	121,602	124,892
Net income per share:
Basic (usd per share)	$ 0.99	$ 1.02	$ 0.98	$ 0.94	$ 0.96	$ 0.96	$ 0.90	$ 0.79	$ 3.93	$ 3.60	$ 2.34
Diluted (usd per share)	$ 0.98	$ 1.01	$ 0.98	$ 0.93	$ 0.95	$ 0.95	$ 0.89	$ 0.78	$ 3.90	$ 3.57	$ 2.33