Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 216,586	$ 185,764	$ 178,468
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	(6,697)	(5,508)	(7,655)
Less: reclassification adjustment for (losses) gains included in net income upon liquidation	(2)	561	0
Foreign currency translation adjustments, before tax	(6,695)	(6,069)	(7,655)
Income tax benefit (expense)	56	11	(78)
Foreign currency translation adjustments, net of tax	(6,639)	(6,058)	(7,733)
Unrealized gains (losses) on investments:
Unrealized gains (losses) arising during period	4	(132)	602
Less: reclassification adjustments for (losses) gains included in net income	(2)	457	7
Changes in unrealized gains (losses) on investments	6	(589)	595
Income tax benefit (expense)	0	256	(283)
Unrealized gains (losses) on investments, net of tax	6	(333)	312
Changes in employee benefit related items:
Amortization of prior service cost	40	(326)	(1,841)
Recognized actuarial (loss) gain	(737)	1,264	(7,486)
Changes in employee benefit related items	(697)	938	(9,327)
Income tax (expense) benefit	(12)	(61)	113
Employee benefit related items, net of tax	(709)	877	(9,214)
Other comprehensive (loss)	(7,342)	(5,514)	(16,635)
Comprehensive income	$ 209,244	$ 180,250	$ 161,833