Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended				12 Months Ended
	Aug. 31, 2018 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016	Dec. 31, 2018 USD ($) Payments	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 31, 2017 USD ($)
Lease Agreements [Member]
Loss Contingencies [Line Items]
Total rent expense					$ 0.5	$ 0.5	$ 0.5
Operating Lease Agreement To Lease Office Space [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date				Apr. 30, 2019
Vancouver Lease [Member] | Vancouver [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date					Feb. 28, 2023
Additional lease term to be extended					5 years
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Aggregate milestone payment					$ 270.0
Upfront payment paid			$ 0.9
Number of milestone payments recorded | Payments					0
Milestone payment					$ 4.0
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Milestone payment								$ 2.0
Upfront payment paid		$ 7.0
Additional milestone payment payable					319.5
Asset Purchase Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid	$ 3.0
Aggregate milestone payment					195.0
Milestone payment					$ 5.0