Condensed Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 66,999	$ 59,828	$ 186,184	$ 158,541
Other comprehensive income:
Foreign currency translation adjustments, before tax	5,872	(4,470)	718	(4,303)
Income tax benefit	38	77	116	24
Foreign currency translation adjustments, net of tax	5,910	(4,393)	834	(4,279)
Changes in employee benefit related items:
Amortization of prior service cost	3	3	10	8
Recognized actuarial gain	149	174	613	374
Changes in employee benefit related items	152	177	623	382
Income tax (expense) benefit	(1)	90	(31)	102
Employee benefit related items, net of tax	151	267	592	484
Other comprehensive income (loss)	6,061	(4,126)	1,426	(3,795)
Comprehensive income	$ 73,060	$ 55,702	$ 187,610	$ 154,746