Commitments and Contingencies - Additional Information (Detail) $ in Millions	1 Months Ended				3 Months Ended		9 Months Ended
	Aug. 31, 2018 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016	Sep. 30, 2018 USD ($) Payments	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Payments	Sep. 30, 2017 USD ($)	Jan. 31, 2017 USD ($)
Lease Agreements [Member]
Loss Contingencies [Line Items]
Total rent expense					$ 0.1	$ 0.1	$ 0.3	$ 0.4
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		5 years
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Aggregate milestone payment							$ 270.0
Milestone payment					$ 4.0		$ 4.0
Upfront payment paid			$ 0.9
Number of milestone payments recorded | Payments					0		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		$ 5.0