Commitments and Contingencies - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended
	Nov. 30, 2017 ft²	Mar. 31, 2019 USD ($) ft² Property Option
Commitments And Contingencies [Line Items]
Tenant improvement allowance | $		$ 6,816
Lessee, Operating Lease, Existence of Option to Extend [true false]		true
Operating lease costs | $		$ 1,200
Variable lease expense | $		200
Cash paid for operating lease liabilities included net cash used in operating activities | $		$ 500
Operating lease, weighted-average remaining lease term		9 years
Operating lease, weighted-average discount rate		6.90%
Richmond Lease [Member]
Commitments And Contingencies [Line Items]
Additional property available under lease | Property		2
Richmond Lease [Member] | Office and Laboratory [Member]
Commitments And Contingencies [Line Items]
Area of space leased		37,900
Richmond Lease [Member] | Operating Lease One [Member] | Research and Office Space [Member]
Commitments And Contingencies [Line Items]
Area of space leased		7,700
Lease expiration date		Aug. 31, 2019
Richmond Lease [Member] | Operating Lease Two [Member]
Commitments And Contingencies [Line Items]
Area of space leased		41,400
Lease expiration date		Dec. 31, 2021
Lease termination date		Mar. 31, 2019
Valbonne Lease [Member]
Commitments And Contingencies [Line Items]
Additional property available under lease | Property		2
Valbonne Lease [Member] | Operating Lease One [Member] | Research and Office Space [Member]
Commitments And Contingencies [Line Items]
Area of space leased		14,036
Lease expiration date		Jun. 30, 2022
Valbonne Lease [Member] | Operating Lease Two [Member] | Office Space
Commitments And Contingencies [Line Items]
Area of space leased		6,800
Lease expiration date		Mar. 31, 2022
Lease commencement date		Apr. 01, 2019
Richmond and Valbonne Leases [Member]
Commitments And Contingencies [Line Items]
Lease, option to extend, description		Certain of these leases also include renewal options at the election of the Company to renew or extend the lease for an additional five to ten years.
Richmond and Valbonne Leases [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Lease agreement, extendable lease term		10 years
Richmond and Valbonne Leases [Member] | Minimum [Member]
Commitments And Contingencies [Line Items]
Lease agreement, extendable lease term		5 years
Brisbane Lease [Member]
Commitments And Contingencies [Line Items]
Area of space leased	87,700
Lease expiration date	May 31, 2029
Remaining commencement, area of space leased		52,620
Expected lease commencement period		2019-06
Tenant improvement allowance | $		$ 6,800
Lease not yet commenced, tenant improvement allowance | $		10,200
Lease not yet commenced, undiscounted lease payments | $		$ 29,300
Number of lease term extension options | Option		2
Brisbane Lease [Member] | Maximum [Member]
Commitments And Contingencies [Line Items]
Lease agreement, extendable lease term		5 years
Brisbane Lease [Member] | Office Space
Commitments And Contingencies [Line Items]
Commencement, area of space leased		35,080
Lease commencement period		2019-01