Summary of Significant Accounting Policies - Additional Information (Detail)		1 Months Ended	12 Months Ended
	Mar. 09, 2018	Sep. 30, 2017 USD ($)	Dec. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)
Significant Accounting Policies [Line Items]
Reverse stock split			the Company effected a reverse split of all shares of its common and preferred stock at a ratio of 1-for-3.96
Reverse stock split ratio	0.2525
Number of operating segments | Segment			1
Number of reportable segments | Segment			1
Deferred offering costs				$ 1,300,000	$ 3,600,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Non-refundable, non-creditable upfront cash payments			$ 35,000,000
Payment for option exercise			$ 3,000,000
Minimum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives			1 year
Impairment charges			$ 0	$ 0
Minimum | Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Payment for option exercise		$ 3,000,000
Maximum
Significant Accounting Policies [Line Items]
Property and equipment, estimated useful lives			5 years
Maximum | Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Payment for option exercise		$ 15,000,000