Commitments and Contingencies - Additional Information (Detail)	1 Months Ended					12 Months Ended
	Dec. 31, 2017 USD ($) Payments	Jan. 31, 2017 USD ($)	Oct. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016	Dec. 31, 2017 USD ($) Payments	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Carna License Agreement [Member]
Loss Contingencies [Line Items]
Upfront payment paid				$ 900,000
Aggregate milestone payment						$ 270,000,000
Number of milestone payments recorded | Payments	0					0
CPF License Agreements [Member]
Loss Contingencies [Line Items]
Upfront payment paid			$ 7,000,000
Aggregate milestone payment		$ 2,000,000
Additional milestone payment payable						$ 319,500,000
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. 27, 2018
Collateralized restricted deposit	$ 13,000					$ 13,000
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
Sublease Agreement [Member] | San Francisco [Member]
Loss Contingencies [Line Items]
Operating lease agreement expiration date						Apr. 30, 2019
Fair value of remaining contractual obligation, net income	$ 100,000					$ 100,000
Lease Agreements [Member]
Loss Contingencies [Line Items]
Total rent expense						$ 500,000	$ 500,000	$ 300,000