CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income (loss)	$ 61,588	$ 119,929	$ (8,508)
Other comprehensive income (loss):
Cumulative translation adjustment	0	(2,257)	(1,333)
Unrealized loss on derivative instruments, net of tax:
Change in fair value of derivative instruments	0	0	(87)
Defined benefit pension and other postretirement benefit plans, net of tax:
Prior service costs arising during the period	(819)	0	(43)
Net gain (loss) arising during the period	4,958	(8,452)	(5,036)
Less: amortization of prior service cost included in net periodic benefit cost	7,084	6,879	5,679
Other comprehensive income (loss)	11,223	(3,830)	(820)
Comprehensive income (loss)	$ 72,811	$ 116,099	$ (9,328)