Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 59,828	$ 65,900	$ 158,541	$ 182,517
Other comprehensive income:
Foreign currency translation adjustment, before reclassification and tax	(4,470)	(1,291)	(4,303)	(5,272)
Less: reclassification adjustment for (losses) included in net income upon liquidation	0	0	0	(36)
Foreign currency translation adjustments, before tax	(4,470)	(1,291)	(4,303)	(5,236)
Income tax benefit	77	26	24	1
Foreign currency translation adjustments, net of tax	(4,393)	(1,265)	(4,279)	(5,235)
Changes in employee benefit related items:
Amortization of prior service cost	3	5	8	6
Recognized actuarial gain	174	295	374	314
Changes in employee benefit related items	177	300	382	320
Income tax benefit (expense)	90	(3)	102	(43)
Employee benefit related items, net of tax	267	297	484	277
Other	0	0	0	133
Other comprehensive (loss) income	(4,126)	(968)	(3,795)	(4,825)
Comprehensive income	$ 55,702	$ 64,932	$ 154,746	$ 177,692