Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)	1 Months Ended	12 Months Ended
	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2015
Significant Accounting Policies [Line Items]
Restricted cash		$ 1,143,000	$ 1,143,000
Allowances for doubtful accounts		0	0
Inventory lower of cost and market adjustment		911,000	1,939,000	$ 2,578,000
Asset retirement obligation		334,000			$ 250,000
Impairment loss recognized		0	0	0
Deferred offering costs		$ 0	0
Wrote off deferred offering costs				$ 2,900,000
Service contract term		12 months
Distribution fee reclassified to deferred revenue		$ 23,389,000	10,433,000
Reserve for deferred costs of revenue		0	0
Excise tax on medical devices				2.30%
Deferred commission		3,500,000	2,600,000
Penalties and interest related to income taxes		0
Operating leases future minimum payments due		2,157,000
ASU No. 2016-18
Significant Accounting Policies [Line Items]
Restricted cash		$ 1,100,000	1,100,000
Itochu Corporation Agreement
Significant Accounting Policies [Line Items]
Remaining term of distribution agreement	8 years 6 months	8 years 6 months
License Cost
Significant Accounting Policies [Line Items]
Estimated useful life		3 years
Distribution Rights | Itochu Corporation Agreement
Significant Accounting Policies [Line Items]
Distribution fee reclassified to deferred revenue	$ 4,000,000	$ 4,000,000
Asset Retirement Obligation
Significant Accounting Policies [Line Items]
Accretion expenses		$ 40,000	36,000	$ 8,000
Minimum
Significant Accounting Policies [Line Items]
Property and equipment, useful life		2 years
Minimum | Patents
Significant Accounting Policies [Line Items]
Estimated useful life		5 years
Maximum
Significant Accounting Policies [Line Items]
Property and equipment, useful life		15 years
Maximum | Patents
Significant Accounting Policies [Line Items]
Estimated useful life		7 years
Irrevocable Standby Letters of Credit
Significant Accounting Policies [Line Items]
Letters of credit drawn, amount		$ 0	0
Collateral for Letters of Credit
Significant Accounting Policies [Line Items]
Restricted cash		900,000	$ 900,000
Collateral For Credit Card
Significant Accounting Policies [Line Items]
Restricted cash		$ 200,000