Commitments and Contingencies - Additional Information (Detail)		1 Months Ended					12 Months Ended
	May 31, 2015 USD ($)	Jul. 31, 2017 USD ($)	Apr. 30, 2017 ft²	Feb. 28, 2017 USD ($)	Jun. 30, 2015 shares	Nov. 30, 2014 USD ($) shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Mar. 31, 2015 USD ($) ft²
Other Commitments [Line Items]
Lease expense, payment due							$ 2,288,000
Rent expense							$ 2,500,000	$ 2,400,000	$ 1,900,000
Standard product warranty, description							Limited one- to five-year warranty
Retainer fees							$ 37,560,000	33,729,000	21,382,000
Common stock, shares issued | shares					2,470,587
Contingent liabilities							0	0
Indemnification Agreement [Member]
Other Commitments [Line Items]
Contingent liabilities							0
Patent Infringement [Member]
Other Commitments [Line Items]
Contingent liabilities							0
Service Agreement [Member]
Other Commitments [Line Items]
Service agreement commitment amount							3,600,000
Licensing Agreements [Member]
Other Commitments [Line Items]
Annual royalty payment							$ 2,500,000	$ 1,900,000	$ 600,000
License agreement commitment amount		$ 500,000
License agreement expiration year		2022
Licensing Agreements [Member] | Mayo And VGL [Member]
Other Commitments [Line Items]
License agreement, terms							The agreement will terminate upon the last to expire patent application, unless terminated earlier. The agreement can be terminated any time after three years from March 2006 by Mayo or VGL.
Licensing Agreements [Member] | Mayo Foundation [Member]
Other Commitments [Line Items]
Retainer fees							$ 40,000
Common stock, shares issued | shares						20,833
Noncash research and development expense						$ 500,000
Licensing Agreements [Member] | Mayo Foundation [Member] | Minimum [Member]
Other Commitments [Line Items]
Annual royalty payment							200,000
Supply Agreements [Member]
Other Commitments [Line Items]
Purchase commitment, amount due in 2018							6,600,000
Purchase commitment, amount due in 2019							6,400,000
Purchase commitment, amount due in 2020							$ 6,200,000
Redwood Office Agreement [Member]
Other Commitments [Line Items]
Lease agreement, commencement period							2015-03
Area of office space | ft²										50,740
Lease agreement, effective date							Jun. 30, 2015
Lease agreement, expiration period							2022-05
Lease expense, payment due										$ 2,000,000
Annual lease expense payable in final year of lease term										$ 2,400,000
Redwood Office Agreement Additional Expansion Premises [Member]
Other Commitments [Line Items]
Area of office space | ft²			8,171					49,980
Lease agreement, effective date			May 31, 2017
Lease expense, payment due								$ 1,200,000
Annual lease expense payable in final year of lease term								$ 2,900,000
Commencement date description							The lease for the Expansion Premises commences on the earlier of (i) the date the Company commences business operations in the Expansion Premises, or (ii) the date upon which the Landlord substantially completes certain improvements to, and permitting for, the Expansion Premises (the Commencement Date). The Commencement Date is expected to occur in the first half of 2018.
Menlo Park Agreement, Original [Member]
Other Commitments [Line Items]
Lease agreement, effective date							May 01, 2010
Lease agreement, expiration date							May 31, 2015
Menlo Park Agreement, Extended [Member]
Other Commitments [Line Items]
Lease agreement, commencement period							2015-03
Lease agreement, expiration date							Sep. 30, 2015
Lease agreement, lease expense	$ 100,000
Facility Lease Agreement, Original [Member]
Other Commitments [Line Items]
Lease agreement, commencement period							2014-08
Lease agreement, effective date							Aug. 21, 2014
Lease agreement, expiration date							May 31, 2015
Facility Lease Agreement, Extended [Member]
Other Commitments [Line Items]
Lease agreement, commencement period							2015-03
Lease agreement, expiration period							2017-02
Lease agreement, lease expense				$ 300,000
Non-cancellable Facility Lease [Member]
Other Commitments [Line Items]
Lease agreement, effective date							Mar. 01, 2017
Lease agreement, expiration date							Feb. 28, 2022
Lease agreement, lease expense							$ 400,000