Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Income	$ 184,665	$ 259,695	$ 98,655	$ 119,098	$ 157,555	$ 30,912	$ 543,015	$ 307,565
Weighted average shares outstanding: [Abstract]
Basic (in shares)	40,101			40,123			40,146	40,073
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	590			716			620	776
Diluted (in shares)	40,691			40,839			40,766	40,849
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in dollars per share)	$ 4.60			$ 2.97			$ 13.53	$ 7.68
Diluted (in dollars per share)	$ 4.54			$ 2.92			$ 13.32	$ 7.53
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0			0			0	0