Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended		12 Months Ended
	Aug. 01, 2016	Nov. 30, 2017	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2016
Stockholders Equity [Line Items]
Sale of common stock, preferred stock, debt securities and warrants				$ 95,700,000
At-The-Market Issuance Sales Agreement, maximum value of common stock to be issued							$ 10,000,000
At-The-Market Issuance Sales Agreement, value of common stock to be issued				$ 5,700,000
Shares of common stock issued during period under At-The-Market Issuance Sales Agreement				1,509,370
Gross proceeds from the sales agreement				$ 4,300,000
Private placement, share lock-up period	2 years
Purchase of technology license with common stock				$ 0	$ 0	$ 775,000
Astellas In-License Agreements [Member] | Collaborative Arrangement [Member]
Stockholders Equity [Line Items]
Restricted stock issued, shares			86,121
Astellas In-License Agreements [Member] | Collaborative Arrangement [Member] | Research and development [Member]
Stockholders Equity [Line Items]
Cash payment for license agreement			$ 250,000
Purchase of technology license with common stock						$ 775,094
Maximum [Member]
Stockholders Equity [Line Items]
Equity position under stock purchase agreement	19.90%
Common Stock [Member]
Stockholders Equity [Line Items]
Shares issued				10,704,000	1,841,000	86,000
Pre-funded warrants to purchase common stock		7,234,285
Pre-funded warrants to purchase common stock, price per share		$ 1.74
Net proceeds from offering		$ 26,400,000
Pre-funded warrants exercise price per share		$ 0.01
Public Offering [Member] | Common Stock [Member]
Stockholders Equity [Line Items]
Shares issued		9,194,286
Public offering, price per share		$ 1.75
Overallotment [Member] | Common Stock [Member]
Stockholders Equity [Line Items]
Shares issued		2,142,857
Public offering, price per share		$ 1.75
Stock Purchase Agreement with AnGes, MG, Inc in Private Placement [Member]
Stockholders Equity [Line Items]
Pre-funded warrants to purchase common stock, price per share	$ 4.24
Number of shares of common stock sold under the sales agreement	1,841,420
Private placement, gross proceeds	$ 7,800,000
Private placement, closing date	Aug. 02, 2016