Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income	$ 117,181	$ 115,242	$ 341,591	$ 320,724
Basic:
Weighted average shares (in shares)	115,181,000	120,115,000	116,203,000	121,112,000
Diluted:
Weighted average shares (in shares)	115,181,000	120,115,000	116,203,000	121,112,000
Dilutive effect of potential common shares (in shares)	687,000	1,328,000	731,000	1,081,000
Diluted weighted average shares (in shares)	115,868,000	121,443,000	116,934,000	122,193,000
Net income per share:
Basic (in usd per share)	$ 1.02	$ 0.96	$ 2.94	$ 2.65
Diluted (in usd per share)	$ 1.01	$ 0.95	$ 2.92	$ 2.62