Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Customer Segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) Customer
Summary Of Significant Accounting Policies [Line Items]
Description of payment of customer invoice contract			The Company invoices its customers based upon the payment terms in the contracts, which is generally from 30 to 60 days.
Contract asset	$ 0		$ 0		$ 0
Revenue recognized, that were included in deferred product revenue			$ 300,000
Application of an optional exemption not to disclose the value of unsatisfied performance obligations			true
Restricted cash	$ 2,500,000		$ 2,500,000
Number of major customers representing outstanding trade receivables | Customer	2		2		3
Protracted length of inventory			1 year
Inventory valuation reserves	$ 300,000		$ 300,000		$ 100,000
Capitalized construction-in-progress cost unpaid			800,000
Accumulated amortization intangible assets	1,600,000		$ 1,600,000		1,500,000
Estimated useful life of intangible assets			10 years
Number of reportable segments | Segment			1
Impairment charges acquired intangible asstes	0	$ 0	$ 0	$ 0
Period of warranty			1 year
Warranty claim liability	0		$ 0		0
Leasehold Improvements
Summary Of Significant Accounting Policies [Line Items]
Capitalized construction-in-progress costs	$ 1,500,000		$ 1,500,000		$ 0
Trade Accounts Receivable | Customer Concentration Risk
Summary Of Significant Accounting Policies [Line Items]
Concentration risk, percentage			61.00%		53.00%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Payment of customer invoice contract period			30 days
Shelf lives of inventory			18 months
Estimated useful life of property and equipment			3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Payment of customer invoice contract period			60 days
Shelf lives of inventory			24 months
Estimated useful life of property and equipment			5 years