Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Earnings Per Share [Abstract]
Net Income	$ 119,098	$ 157,555	$ 30,912	$ 79,525	$ 131,390	$ 32,637	$ 307,565	$ 243,551
Weighted average shares outstanding: [Abstract]
Basic (in shares)	40,123			39,933			40,073	39,750
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	716			932			776	1,061
Diluted (in shares)	40,839			40,865			40,849	40,811
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in dollars per share)	$ 2.97			$ 1.99			$ 7.68	$ 6.13
Diluted (in dollars per share)	$ 2.92			$ 1.95			$ 7.53	$ 5.97
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0			0			0	1