Significant Accounting Policies - Additional Information (Detail) $ in Millions		9 Months Ended
	Mar. 09, 2018	Sep. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jun. 29, 2017 USD ($)
Significant Accounting Policies [Line Items]
Description to be remained in emerging growth company		The Company will remain an emerging growth company until the earliest of (1) the last day of its first fiscal year (a) following the fifth anniversary of the completion of its initial public offering, (b) in which the Company has a total annual gross revenue of at least $1.07 billion, or (c) in which the Company is deemed to be a large accelerated filer, which means the market value of the common stock that is held by non-affiliates exceeds $700.0 million of the prior June 30th and (2) the date on which the Company has issued more than $1.0 billion in non-convertible debt securities during the prior three-year period.
Minimum annual gross revenue in order to remain classified as emerging growth company		$ 1,070.0
Maximum market value of common stock held by non-affiliates to be remained in emerging growth company					$ 700.0
Additional non-convertible debt securities to be remained in emerging growth company		$ 1,000.0
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
Deferred offering costs			$ 3.6	$ 1.3
Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Non-refundable, non-creditable upfront cash payments		$ 35.0
Payment for option exercise		$ 3.0