Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 46,439	$ 58,160
Other comprehensive income:
Foreign currency translation adjustments, before tax	1,152	3,448
Income tax expense	(2)	(20)
Foreign currency translation adjustments, net of tax	1,150	3,428
Changes in employee benefit related items:
Amortization of prior service cost	5	0
Recognized actuarial gain (loss)	286	(38)
Changes in employee benefit related items	291	(38)
Income tax benefit (expense)	2	(41)
Employee benefit related items, net of tax	293	(79)
Other	0	(1)
Other comprehensive income	1,443	3,348
Comprehensive income	$ 47,882	$ 61,508