Consolidated 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
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 20,619	$ 13,786	$ 37,384	$ 41,427
Other comprehensive income (loss):
Foreign currency translation adjustment	1,581	(193)	3,351	10,638
Adjustment to retirement benefit liability, net of tax	(2)	61	(8)	59
Adjustment to post employment health care benefit liability, net of tax	29	38	88	114
Other comprehensive income (loss)	1,608	(94)	3,431	10,811
Total comprehensive income	$ 22,227	$ 13,692	$ 40,815	$ 52,238