Stockholders' Equity - Additional Information (Detail) - USD ($)	1 Months Ended	3 Months Ended
	Oct. 31, 2018	Dec. 31, 2018	Sep. 30, 2018
Class Of Stock [Line Items]
Capital stock authorized for issuance		150,000,000
Common stock, shares authorized		145,000,000	145,000,000
Common stock, par value		$ 0.001	$ 0.001
Preferred stock, shares authorized		5,000,000
Preferred stock, par value		$ 0.001
Common stock, shares outstanding		92,591,457	88,505,302
Proceeds from the issuance of common stock		$ 60,521,729
Janssen and JJDC | Common Stock Purchase Agreement
Class Of Stock [Line Items]
Shares issued	3,260,869
Proceeds from the issuance of common stock	$ 75,000,000
Stock issued, price per share	$ 23.00
2004 Equity Incentive Plan, 2013 Equity Incentive Plan, and Inducement Grants
Class Of Stock [Line Items]
Common Stock, Share reserve for issuance		7,954,283