Schedule of Impact of ASC 606 Adoption on Condensed Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Deferred revenue, current portion	$ 1,342		$ 395
Deferred revenue, net of current portion	3,749		1,302
Accumulated deficit	(594,664)		$ (586,969)
ASC 606
Significant Accounting Policies [Line Items]
Deferred revenue, current portion		$ 1,027
Deferred revenue, net of current portion		3,381
Accumulated deficit		(589,680)
ASC 606 | Adjustments
Significant Accounting Policies [Line Items]
Deferred revenue, current portion	947	632
Deferred revenue, net of current portion	2,645	2,079
Accumulated deficit	(3,592)	(2,711)
ASC 606 | Balances without adoption of ASC Topic 606
Significant Accounting Policies [Line Items]
Deferred revenue, current portion	395	395
Deferred revenue, net of current portion	1,104	1,302
Accumulated deficit	$ (591,072)	$ (586,969)