Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Earnings Per Share [Abstract]
Net Income	$ 259,695	$ 98,655	$ 157,555	$ 30,912	$ 358,350	$ 188,467
Weighted average shares outstanding: [Abstract]
Basic (in shares)	40,125		39,973		40,169	40,049
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	620		742		631	788
Diluted (in shares)	40,745		40,715		40,800	40,837
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in dollars per share)	$ 6.47		$ 3.94		$ 8.92	$ 4.71
Diluted (in dollars per share)	$ 6.37		$ 3.87		$ 8.78	$ 4.62
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0		0		0	0