Revenue from Contracts with Customers - Schedule of Impact on Consolidated Balance Sheet of Adopting ASC 606 (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Current liabilities:
Current portion of deferred revenue	$ 1,517		$ 1,573
Total current liabilities	17,016		15,798
Long-term liabilities:
Deferred revenue, less current portion	15,029		16,759
Total long-term liabilities	15,177		17,069
Total liabilities	32,193		32,867
Stockholders’ equity:
Accumulated deficit	(420,586)		(366,111)
Total stockholders’ equity	64,274		104,319
Total liabilities and stockholders’ equity	$ 96,467		$ 137,186
Accounting Standards Update 2014-09 [Member]
Current liabilities:
Current portion of deferred revenue		$ 1,517
Total current liabilities		15,742
Long-term liabilities:
Deferred revenue, less current portion		16,166
Total long-term liabilities		16,476
Total liabilities		32,218
Stockholders’ equity:
Accumulated deficit		(365,462)
Total stockholders’ equity		104,968
Total liabilities and stockholders’ equity		137,186
Accounting Standards Update 2014-09 [Member] | Adjustments Due to ASC 606 [Member]
Current liabilities:
Current portion of deferred revenue		(56)
Total current liabilities		(56)
Long-term liabilities:
Deferred revenue, less current portion		(593)
Total long-term liabilities		(593)
Total liabilities		(649)
Stockholders’ equity:
Accumulated deficit		649
Total stockholders’ equity		$ 649