Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 58,457	$ 41,741	$ 116,617	$ 85,651
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	(7,429)	2,643	(3,981)	5,842
Less: reclassification adjustment for (losses) included in net income upon liquidation	(36)	0	(36)	0
Foreign currency translation adjustments, before tax	(7,393)	2,643	(3,945)	5,842
Income tax (expense) benefit	(5)	97	(25)	52
Foreign currency translation adjustments, net of tax	(7,398)	2,740	(3,970)	5,894
Unrealized gains on investments:
Unrealized gains arising during period	0	9	0	4
Less: reclassification adjustments for gains included in net income	0	0	0	0
Changes in unrealized gains on investments	0	9	0	4
Income tax benefit	0	3	0	2
Unrealized gains on investments, net of tax	0	12	0	6
Changes in employee benefit related items:
Amortization of prior service cost	1	8	1	10
Recognized actuarial gain	57	421	19	533
Changes in employee benefit related items	58	429	20	543
Income tax benefit (expense)	1	(4)	(40)	(31)
Employee benefit related items, net of tax	59	425	(20)	512
Other	134	0	133	0
Other comprehensive (loss) income	(7,205)	3,177	(3,857)	6,412
Comprehensive income	$ 51,252	$ 44,918	$ 112,760	$ 92,063