Research and License Agreements - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Oct. 31, 2017 USD ($)	Jan. 31, 2017 USD ($) LicenseAgreement	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2013 USD ($)
Contract Research Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Convertible preferred stock, shares issued | shares			0	0
Contract Research Agreement | Series B Convertible Preferred Stock
Research And Development Arrangement Contract To Perform For Others [Line Items]
Convertible preferred stock, shares issued | shares					70,028
Convertible preferred stock, fair values					$ 1,100,000
University Research Agreement
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research agreement date			2013-12
Maximum allowable research and development expenses per year			$ 2,500,000			$ 386,000
Research agreement expiration date			Aug. 31, 2018
Increasing the maximum research expenditure limit	$ 375,000
Payments pursuant to research agreements	$ 188,000		$ 563,000	$ 832,000	563,000
License Agreements
Research And Development Arrangement Contract To Perform For Others [Line Items]
Research agreement date			2013-12
Number of license agreements | LicenseAgreement		2
Maximum future contingent license payment		$ 6,400,000
Aggregate potential milestone payments for receipt of regulatory approval		5,000,000
Aggregate potential milestone payments for final regulatory approval of second indication		$ 500,000
Annual license fees paid			$ 30,000	$ 10,000	$ 0
License Agreements | Minimum [Member]
Research And Development Arrangement Contract To Perform For Others [Line Items]
Rate of revenue share		6.50%
License Agreements | Maximum
Research And Development Arrangement Contract To Perform For Others [Line Items]
Rate of revenue share		25.00%