Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 50,258	$ 42,086	$ 143,922	$ 134,677
Other comprehensive income (loss):
Foreign currency translation adjustments, before reclassification and tax	320	(2,329)	715	(4,014)
Less: reclassification adjustment for (losses) gains included in net income upon liquidation	(2)	0	(2)	563
Foreign currency translation adjustments, before tax	322	(2,329)	717	(4,577)
Income tax benefit	29	47	86	48
Foreign currency translation adjustments, net of tax	351	(2,282)	803	(4,529)
Unrealized gains (losses) on investments:
Unrealized gains (losses) arising during period	5	(13)	(2)	(137)
Less: reclassification adjustments for (losses) gains included in net income	(3)	455	(4)	455
Changes in unrealized gains (losses) on investments	8	(468)	2	(592)
Income tax (expense) benefit	(2)	156	2	258
Unrealized gains (losses) on investments, net of tax	6	(312)	4	(334)
Changes in employee benefit related items:
Amortization of prior service cost	5	0	41	0
Recognized actuarial gain	265	407	608	584
Changes in employee benefit related items	270	407	649	584
Income tax benefit (expense)	7	(11)	(23)	(35)
Employee benefit related items, net of tax	277	396	626	549
Other comprehensive income (loss)	634	(2,198)	1,433	(4,314)
Comprehensive income	$ 50,892	$ 39,888	$ 145,355	$ 130,363