Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 56,926	$ 52,274	$ 119,185	$ 98,713
Other comprehensive income:
Foreign currency translation adjustments, before tax	2,473	(985)	(5,154)	167
Income tax benefit (expense)	72	(51)	78	(53)
Foreign currency translation adjustments, net of tax	2,545	(1,036)	(5,076)	114
Changes in employee benefit related items:
Amortization of prior service cost	4	0	7	5
Recognized actuarial gain (loss)	295	(86)	464	200
Changes in employee benefit related items	299	(86)	471	205
Income tax (expense) benefit	(2)	10	(30)	12
Employee benefit related items, net of tax	297	(76)	441	217
Other comprehensive income (loss)	2,842	(1,112)	(4,635)	331
Comprehensive income	$ 59,768	$ 51,162	$ 114,550	$ 99,044