Debt, Commitments and Contingencies - Additional Information (Detail) - USD ($)	1 Months Ended								12 Months Ended
	May. 31, 2015	Jan. 31, 2015	Jun. 30, 2014	Jul. 31, 2013	Aug. 31, 2012	Apr. 30, 2012	Dec. 31, 2011	Oct. 31, 2007	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Noncancelable operating lease expiration period									2017-05
Extended term of lease							5 years
Rent expense									$ 400,000	$ 200,000	$ 200,000
Convertible promissory note	$ 2,000,000								2,000,000		500,000
Accrued interest on convertible promissory note	$ 500,000								925,000	415,000	254,000
Research and development expenses									34,504,000	16,777,000	13,832,000
The Scripps Research Institute [Member] | Maximum [Member]
Debt Instrument [Line Items]
Obligation to pay an aggregate amount under agreement								$ 2,750,000
Research Funding and Option Agreement [Member] | The Scripps Research Institute [Member]
Debt Instrument [Line Items]
Agreement renewal period								12 months
Written notice period to terminate the agreement								30 days
Issue of common stock, shares in consideration for adjustment of sublicense payments		119,840
Fair value of common stock issued		$ 1,400,000
Research and development expenses									700,000	700,000	600,000
Research Funding and Option Agreement [Member] | National Foundation for Cancer Research [Member]
Debt Instrument [Line Items]
Charitable donations									400,000	$ 400,000	$ 400,000
Master Services Agreement [Member]
Debt Instrument [Line Items]
Research and development expenses									$ 5,100,000
Milestones payment terms									up to the mid seven figures.
Silicon Valley Bank [Member]
Debt Instrument [Line Items]
Borrowings under loan and security agreement			$ 5,000,000	$ 5,000,000	$ 1,250,000	$ 1,250,000			$ 5,202,000
Debt instrument interest rate percentage			5.88%	5.00%	4.85%	4.89%
Increase in available credit under loan and security agreement				$ 10,000,000
Cash proceeds after repayment of debt				$ 2,900,000
Debt instrument fee									$ 500,000
Maturity date			Jun. 30, 2017		Dec. 31, 2015	Dec. 31, 2015
Silicon Valley Bank [Member] | 4.89% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments through December 2012 and, beginning in December 2013, monthly payments of principal and interest through the maturity date in December 2015.
Silicon Valley Bank [Member] | 4.85% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments through December 2012 and, beginning in December 2013, monthly payments of principal and interest through the maturity date in December 2015.
Silicon Valley Bank [Member] | 5.0% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments on each $5.0 million borrowing through June 2014 and, beginning in July 2014, monthly payments of principal and interest through the maturity date in June 2017.
Silicon Valley Bank [Member] | 5.88% Commercial Bank Debt [Member]
Debt Instrument [Line Items]
Debt instrument payment terms									We were obligated to make interest-only payments on each $5.0 million borrowing through June 2014 and, beginning in July 2014, monthly payments of principal and interest through the maturity date in June 2017.
Silicon Valley Bank [Member] | Series D Redeemable Convertible Preferred Stock [Member]
Debt Instrument [Line Items]
Warrant issued to purchase shares				59,312
Exercise price of warrant issued				$ 2.529
Warrant exercisable			118,624
Warrants issued upon conversion	14,913
Subordinated Unsecured Convertible Promissory Note [Member]
Debt Instrument [Line Items]
Debt instrument interest rate percentage							8.00%
Convertible promissory note borrowed							$ 2,000,000