Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 8,352	$ 3,996
Other comprehensive income (loss):
Foreign currency translation adjustment	110	679
Cumulative effect of accounting change	(350)	0
Adjustment to retirement benefit liability, net of tax	(5)	(3)
Adjustment to post employment health care benefit liability, net of tax	32	29
Other comprehensive income (loss)	(213)	705
Total comprehensive income	$ 8,139	$ 4,701