Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 19,503	$ 20,619	$ 50,692	$ 37,384
Other comprehensive income (loss):
Foreign currency translation adjustment	139	1,581	(1,538)	3,351
Cumulative effect of accounting change (see Note 2)	0	0	(350)	0
Adjustment to retirement benefit liability, net of tax	(5)	(2)	(15)	(8)
Adjustment to post employment health care benefit liability, net of tax	32	29	95	88
Other comprehensive income (loss)	166	1,608	(1,808)	3,431
Total comprehensive income	$ 19,669	$ 22,227	$ 48,884	$ 40,815