Net Income Per Share - Calculation of Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net income	$ 46,196	$ 89,915	$ 114,612	$ 109,798	$ 136,111	$ 224,410
Basic:
Weighted average shares (in shares)	112,865,000		116,381,000		113,026,000	116,722,000
Diluted:
Weighted average shares (in shares)	112,865,000		116,381,000		113,026,000	116,722,000
Dilutive effect of potential common shares (in shares)	256,000		607,000		463,000	753,000
Diluted weighted average shares (in shares)	113,121,000		116,988,000		113,489,000	117,475,000
Net income per share:
Basic (in usd per share)	$ 0.41		$ 0.98		$ 1.20	$ 1.92
Diluted (in usd per share)	$ 0.41		$ 0.98		$ 1.20	$ 1.91