CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (11,694)	$ (11,498)	$ (47,928)	$ (50,717)	$ (61,662)
Other comprehensive income (loss), net of tax:
Change in foreign currency translation adjustments	(2)	(2)	19	8	8
Pension benefit plan	147	279	197	413	532
Total comprehensive income (loss)	$ (11,549)	$ (11,221)	$ (47,712)	$ (50,296)	$ (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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