Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 242,397	$ 207,422	$ 216,586
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	(6,884)	9,671	(6,697)
Less: reclassification adjustment for (losses) included in net income upon liquidation	(36)	0	(2)
Foreign currency translation adjustments, before tax	(6,848)	9,671	(6,695)
Income tax benefit	217	3	56
Foreign currency translation adjustments, net of tax	(6,631)	9,674	(6,639)
Unrealized gains on investments:
Unrealized gains arising during period	0	2	4
Less: reclassification adjustments for losses included in net income	0	0	(2)
Changes in unrealized gains on investments	0	2	6
Income tax benefit	0	2	0
Unrealized gains on investments, net of tax	0	4	6
Changes in employee benefit related items:
Amortization of prior service cost	8	9	40
Recognized actuarial gain (loss)	541	(1,115)	(737)
Changes in employee benefit related items	549	(1,106)	(697)
Income tax expense	(49)	(10)	(12)
Employee benefit related items, net of tax	500	(1,116)	(709)
Other	133	0	0
Other comprehensive (loss) income	(5,998)	8,562	(7,342)
Comprehensive income	$ 236,399	$ 215,984	$ 209,244