Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ 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	$ 157,555	$ 30,912	$ 131,390	$ 32,637	$ 188,467	$ 164,027
Weighted average shares outstanding: [Abstract]
Basic (in shares)	39,973		39,827		40,049	39,654
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	742		1,021		788	1,119
Diluted (in shares)	40,715		40,848		40,837	40,773
Earnings Per Share, Basic and Diluted [Abstract]
Basic (in dollars per share)	$ 3.94		$ 3.30		$ 4.71	$ 4.14
Diluted (in dollars per share)	$ 3.87		$ 3.22		$ 4.62	$ 4.02
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	0		50		0	50