Commitments and Contingencies - Summary of Deferred Revenue (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Deferred Revenue Arrangement [Line Items]
Total deferred revenue	$ 2,467	$ 1,304
Current maturities of deferred revenue	(1,540)	(1,304)
Deferred revenue long-term	927
Product [Member]
Deferred Revenue Arrangement [Line Items]
Current maturities of deferred revenue	(1,431)	$ (1,304)
Supply Agreement [Member]
Deferred Revenue Arrangement [Line Items]
Current maturities of deferred revenue	$ (109)