CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (15,510)	$ (21,174)	$ (36,234)	$ (39,219)	$ (61,662)
Other comprehensive income (loss), net of tax:
Change in foreign currency translation adjustments	11	1	21	10	8
Pension benefit plan	44	183	49	134	532
Total comprehensive loss	$ (15,455)	$ (20,990)	$ (36,164)	$ (39,075)	$ (61,122)

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

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