Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 238,563	$ 242,397	$ 207,422
Other comprehensive (loss) income:
Foreign currency translation adjustments, before reclassification and tax	1,900	(6,884)	9,671
Less: reclassification adjustment for (losses) included in net income upon liquidation	0	(36)	0
Foreign currency translation adjustments, before tax	1,900	(6,848)	9,671
Income tax (expense) benefit	(161)	217	3
Foreign currency translation adjustments, net of tax	1,739	(6,631)	9,674
Unrealized gains on investments:
Unrealized gains arising during period	0	0	2
Income tax benefit	0	0	2
Unrealized gains on investments, net of tax	0	0	4
Changes in employee benefit related items:
Amortization of prior service cost	6	8	9
Recognized actuarial (loss) gain	(3,011)	541	(1,115)
Changes in employee benefit related items	(3,005)	549	(1,106)
Income tax benefit (expense)	99	(49)	(10)
Employee benefit related items, net of tax	(2,906)	500	(1,116)
Other	0	133	0
Other comprehensive (loss) income	(1,167)	(5,998)	8,562
Comprehensive income	$ 237,396	$ 236,399	$ 215,984