Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Other Comprehensive Income (Loss), Net of Tax [Abstract]
Net income (loss)	$ (7,752)	$ 13,786	$ 18,612	$ 9,029	$ 220	$ 13,632	$ 4,087	$ 5,509	$ 33,675	$ 23,448	$ (81,824)
Other comprehensive income (loss):
Foreign currency translation adjustment									6,945	(6,228)	(4,364)
Reclassification of loss on cash flow hedges, net of tax									0	143	(143)
Adjustment to retirement benefit liability, net of tax									55	49	(24)
Adjustment to post-retirement healthcare benefit liability, net of tax									695	171	(1,435)
Other comprehensive income (loss)									7,695	(5,865)	(5,966)
Total comprehensive income (loss)									$ 41,370	$ 17,583	$ (87,790)