Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income	$ 25,176	$ 20,619	$ 12,769	$ 3,996	$ (7,752)	$ 13,786	$ 18,612	$ 9,029	$ 62,560	$ 33,675	$ 23,448
Other comprehensive income (loss):
Foreign currency translation adjustment									3,150	6,945	(6,228)
Reclassification of loss on cash flow hedges, net of tax									0	0	143
Adjustment to retirement benefit liability, net of tax									(9)	55	49
Adjustment to post-retirement healthcare benefit liability, net of tax									214	695	171
Other comprehensive income (loss)									3,355	7,695	(5,865)
Total comprehensive income									$ 65,915	$ 41,370	$ 17,583