Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 58,160	$ 43,910
Other comprehensive income (loss):
Foreign currency translation adjustments, before tax	3,448	3,199
Income tax (expense)	(20)	(45)
Foreign currency translation adjustments, net of tax	3,428	3,154
Unrealized (losses) on investments:
Unrealized (losses) arising during period	0	(5)
Less: reclassification adjustments for (losses) included in net income	0	0
Changes in unrealized (losses) on investments	0	(5)
Income tax benefit (expense)	(1)	(1)
Unrealized (losses) on investments, net of tax	(1)	(6)
Changes in employee benefit related items:
Amortization of prior service cost	0	2
Recognized actuarial (loss) gain	(38)	112
Changes in employee benefit related items	(38)	114
Income tax (expense)	(41)	(27)
Employee benefit related items, net of tax	(79)	87
Other comprehensive income	3,348	3,235
Comprehensive income	$ 61,508	$ 47,145