Commitments and Contingencies - Additional Information (Detail)	1 Months Ended				12 Months Ended
	Oct. 31, 2018 USD ($) ft²	Mar. 31, 2017 USD ($) ft² Lease	Dec. 31, 2015 ft²	Mar. 31, 2014 ft²	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss Contingencies [Line Items]
Number of lease renewal option | Lease		1
Operating lease payments					$ 17,420,000
Rent expense					300,000	$ 1,500,000
Other potential commitments					10,000,000	10,000,000
Pending or threatened legal proceedings					$ 0
Percentag of contingent cash payment with remainder on future milestone payments					7.50%
Contingent Payable					$ 264,000
Maximum [Member]
Loss Contingencies [Line Items]
Commitment to make development and sales-related milestone payments					$ 30,000,000	$ 30,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Proceeds from future licensing transaction percentage					10.00%
Menlo Park, California [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²				12,900
Operating facility lease term				39 months
Lease expiration end date				2017-08
Menlo Park, California [Member] | Sublease Agreement [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²			10,900
Operating facility lease term			24 months
Lease expiration end date			2017-12
1020 Marsh Road, Menlo Park, California [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²		34,500
Operating facility lease term		86 months
Lease commencement date		August 2017
Lease agreement, one renewal option term		5 years
Operating lease payments		$ 20,000,000			$ 17,000,000
2479 E. Bayshore Blvd, Palo Alto, California [Member] | Sublease Agreement [Member]
Loss Contingencies [Line Items]
Operating facility lease area | ft²	4,240
Lease commencement date	October 2018
Lease expires date	Aug. 31, 2020
Sublease payment receivable	$ 300,000