CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (LOSS) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net loss	$ (11,498)	$ (33,628)	$ (50,717)	$ (109,677)	$ (131,845)
Other comprehensive income (loss), net of tax :
Change in foreign currency translation adjustments	(2)	8	8	64	10
Pension benefit plan	279	(409)	413	(644)	(388)
Total comprehensive loss	$ (11,221)	$ (34,029)	$ (50,296)	$ (110,257)	$ (132,223)