Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net income	$ 89,915	$ 109,798
Basic:
Weighted average shares (in shares)	113,187,000	117,068,000
Diluted:
Weighted average shares (in shares)	113,187,000	117,068,000
Dilutive effect of potential common shares (in shares)	671,000	898,000
Diluted weighted average shares (in shares)	113,858,000	117,966,000
Net income per share:
Basic (in usd per share)	$ 0.79	$ 0.94
Diluted (in usd per share)	$ 0.79	$ 0.93