Revenue from Contracts with Customers - Impact of Adoption of Asc 606 on Company's Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	$ 12,568		$ 8,852
Accumulated deficit	(344,280)		$ (305,056)
Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net		$ 8,987
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	12,197
Accumulated deficit	(344,651)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Accounts receivable, net	371
Accumulated deficit	$ 371