Commitments and Contingencies - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Loss Contingencies [Line Items]
Operating lease expiration year	2023
Rent expense	$ 1,700,000	$ 1,600,000
Tenant improvement allowances	0	0
Restricted cash	2,332,000	1,898,000	$ 2,284,000	$ 1,884,000
Purchase commitments	$ 2,500,000		2,600,000
Period of purchase commitment of inventory from specified date	6 months
Loss contingency accrual	$ 0		$ 0
Cancelable Lease Agreement [Member]
Loss Contingencies [Line Items]
Motor vehicle lease expenses	1,200,000	$ 700,000
Maximum [Member] | Cancelable Lease Agreement [Member]
Loss Contingencies [Line Items]
Penalties	$ 100,000