Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share [Abstract]
Net Income (loss)	$ 131,390	$ 32,637	$ 117,049	$ 31,339	$ 164,027	$ 148,388
Weighted average shares outstanding: [Abstract]
Basic (in shares)	39,827		40,453		39,654	40,413
Effect of dilutive securities: [Abstract]
Stock options and employee stock purchase plan (in shares)	1,021		1,361		1,119	1,427
Diluted (in shares)	40,848		41,814		40,773	41,840
Basic (in dollars per share)	$ 3.30		$ 2.89		$ 4.14	$ 3.67
Diluted (in dollars per share)	$ 3.22		$ 2.80		$ 4.02	$ 3.55
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares)	50		0		50	0