Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended					3 Months Ended
	Mar. 31, 2018 Payments	Jan. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016	Mar. 31, 2018 USD ($) Payments	Mar. 31, 2017 USD ($)
Lease Agreements [Member]
Loss Contingencies [Line Items]
Total rent expense						$ 0.1	$ 0.1
Operating Lease Agreement To Lease Office Space [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date					Apr. 30, 2019
Operating Lease Agreement To Sublease Office Space [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date						Feb. 28, 2018
Vancouver Lease [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Restricted deposit expiration date						Feb. 28, 2018
Operating lease agreement expiration date	Feb. 28, 2023
Additional lease term to be extended	5 years
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid				$ 0.9
Aggregate milestone payment						$ 270.0
Number of milestone payments recorded | Payments	0					0
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Upfront payment paid			$ 7.0
Aggregate milestone payment		$ 2.0
Additional milestone payment payable						$ 319.5