Commitments and Contingencies - Additional Information (Detail)					12 Months Ended
	Apr. 14, 2016 DerivativeLawsuit	Apr. 02, 2016 USD ($)	Jun. 07, 2012 USD ($)	Dec. 07, 2011 USD ($)	Dec. 31, 2016 USD ($) LegalProceeding	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Jul. 31, 2015 ft²	May 19, 2014 USD ($) ft²	Mar. 18, 2014 USD ($) ft²	Dec. 01, 2013 USD ($) ft²	Nov. 28, 2012 USD ($) ft²
Commitments And Contingencies Disclosure [Line Items]
Existing square feet of leased property | ft²									9,560	22,775	16,750	13,250
License agreement, expected milestone payment					$ 187,500,000
Company obligations for potential milestone payments for CRO Contracts					$ 22,700,000
Number legal proceedings | LegalProceeding					4
Number of shareholders derivative lawsuits | DerivativeLawsuit	2
Los Angeles
Commitments And Contingencies Disclosure [Line Items]
Office lease, lease term		10 years
Additional lease rent per month		$ 150,000
Lease expiration date					Mar. 31, 2026
Additional square feet leased | ft²								26,000
Lease commencement date					Apr. 01, 2016
Percentage of increase in annual rent		3.00%
South San Francisco
Commitments And Contingencies Disclosure [Line Items]
Office lease, lease term		10 years
Additional lease rent per month		$ 51,400
Lease expiration date					Mar. 31, 2026
Additional square feet leased | ft²								13,000
Lease commencement date					Apr. 01, 2016
Percentage of increase in annual rent		3.00%
Additional Office Space Lease
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month									$ 22,886	$ 11,487	$ 10,400	$ 12,145
Additional square feet leased | ft²									7,152	2,908	5,949	3,500
Minimum | CRO services
Commitments And Contingencies Disclosure [Line Items]
Written notice of termination, period					30 days
Maximum | CRO services
Commitments And Contingencies Disclosure [Line Items]
Written notice of termination, period					45 days
Maximum | Additional Office Space Lease
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month									$ 27,328	$ 12,928	$ 25,100	$ 14,080
Lease Agreement executed on December 7, 2011
Commitments And Contingencies Disclosure [Line Items]
Office lease, lease term				7 years
Lease expiration date					Dec. 09, 2018
Office space, lease extension term				5 years
Office space, lease extension Stand-by letter of credit				$ 2,500,000
Rent expense					$ 3,547,300	$ 1,597,200	$ 1,126,700
Lease Agreement executed on December 7, 2011 | Minimum
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month				44,400
Lease Agreement executed on December 7, 2011 | Maximum
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month				$ 53,000
Lease Agreement executed on June 7, 2012
Commitments And Contingencies Disclosure [Line Items]
Office lease, lease term			7 years
Office space, lease extension term			5 years
Office space, lease extension Stand-by letter of credit			$ 1,591,400
Lease Agreement executed on June 7, 2012 | Minimum
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month			20,250
Lease Agreement executed on June 7, 2012 | Maximum
Commitments And Contingencies Disclosure [Line Items]
Additional lease rent per month			$ 30,820