Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 43,178	$ 50,258	$ 128,829	$ 143,922
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	2,672	320	8,514	715
Less: reclassification adjustment for (losses) included in net income upon liquidation	0	(2)	0	(2)
Foreign currency translation adjustments, before tax	2,672	322	8,514	717
Income tax (expense) benefit	(2)	29	50	86
Foreign currency translation adjustments, net of tax	2,670	351	8,564	803
Unrealized (losses) gains on investments:
Unrealized (losses) gains arising during period	(2)	5	2	(2)
Less: reclassification adjustments for (losses) included in net income	0	(3)	0	(4)
Changes in unrealized (losses) gains on investments	(2)	8	2	2
Income tax benefit (expense)	2	(2)	4	2
Unrealized gains on investments, net of tax	0	6	6	4
Changes in employee benefit related items:
Amortization of prior service cost	2	5	12	41
Recognized actuarial loss	88	265	621	608
Changes in employee benefit related items	90	270	633	649
Income tax benefit (expense)	0	7	(31)	(23)
Employee benefit related items, net of tax	90	277	602	626
Other comprehensive income	2,760	634	9,172	1,433
Comprehensive income	$ 45,938	$ 50,892	$ 138,001	$ 145,355