Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 207,422	$ 216,586	$ 185,764
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	9,671	(6,697)	(5,508)
Less: reclassification adjustment for (losses) gains included in net income upon liquidation	0	(2)	561
Foreign currency translation adjustments, before tax	9,671	(6,695)	(6,069)
Income tax (expense) benefit	3	56	11
Foreign currency translation adjustments, net of tax	9,674	(6,639)	(6,058)
Unrealized gains (losses) on investments:
Unrealized gains (losses) arising during period	2	4	(132)
Less: reclassification adjustments for (losses) gains included in net income	0	(2)	457
Changes in unrealized gains (losses) on investments	2	6	(589)
Income tax benefit	2	0	256
Unrealized gains (losses) on investments, net of tax	4	6	(333)
Changes in employee benefit related items:
Amortization of prior service cost	9	40	(326)
Recognized actuarial (loss) gain	(1,115)	(737)	1,264
Changes in employee benefit related items	(1,106)	(697)	938
Income tax (expense)	(10)	(12)	(61)
Employee benefit related items, net of tax	(1,116)	(709)	877
Other comprehensive income (loss)	8,562	(7,342)	(5,514)
Comprehensive income	$ 215,984	$ 209,244	$ 180,250