Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 22,837	$ 12,769	$ 31,189	$ 16,765
Other comprehensive (loss) income:
Foreign currency translation adjustment	(1,787)	1,091	(1,677)	1,770
Cumulative effect of accounting change (see Note 2)	0	0	(350)	0
Adjustment to retirement benefit liability, net of tax	(5)	(3)	(10)	(6)
Adjustment to post employment health care benefit liability, net of tax	31	30	63	59
Other comprehensive (loss) income	(1,761)	1,118	(1,974)	1,823
Total comprehensive income	$ 21,076	$ 13,887	$ 29,215	$ 18,588