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, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income	$ 94,416	$ 75,749	$ 46,196	$ 89,915	$ 112,842	$ 117,181	$ 114,612	$ 109,798	$ 306,276	$ 454,433	$ 434,288
Basic:
Weighted average shares (in shares)									112,729	115,656	120,513
Diluted:
Weighted average shares (in shares)									112,729	115,656	120,513
Dilutive effect of potential common shares (in shares)									589	755	1,089
Diluted weighted average shares (in shares)									113,318	116,411	121,602
Net income per share:
Basic (usd per share)	$ 0.84	$ 0.67	$ 0.41	$ 0.79	$ 0.99	$ 1.02	$ 0.98	$ 0.94	$ 2.72	$ 3.93	$ 3.60
Diluted (usd per share)	$ 0.84	$ 0.67	$ 0.41	$ 0.79	$ 0.98	$ 1.01	$ 0.98	$ 0.93	$ 2.70	$ 3.90	$ 3.57