Property and Equipment - (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 31,305	$ 26,974
Less: Accumulated depreciation and amortization	(17,248)	(14,816)
Total property and equipment, net	$ 14,057	12,158
Manufacturing equipment
Property, Plant and Equipment [Line Items]
Useful Life (Years)	5 years
Property and equipment, gross	$ 8,395	6,445
Lease and loaner instruments
Property, Plant and Equipment [Line Items]
Property and equipment, gross	4,106	3,581
Less: Accumulated depreciation and amortization	$ (1,900)	(1,500)
Lease and loaner instruments | Minimum
Property, Plant and Equipment [Line Items]
Useful Life (Years)	1 year
Lease and loaner instruments | Maximum
Property, Plant and Equipment [Line Items]
Useful Life (Years)	5 years
Prototype instruments
Property, Plant and Equipment [Line Items]
Useful Life (Years)	2 years
Property and equipment, gross	$ 2,938	2,975
Computer equipment
Property, Plant and Equipment [Line Items]
Useful Life (Years)	3 years
Property and equipment, gross	$ 2,067	1,592
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Useful Life (Years)	5 years
Property and equipment, gross	$ 1,670	1,642
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	11,971	8,878
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 158	$ 1,861