Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		6 Months Ended	12 Months Ended
	Oct. 31, 2018	Oct. 31, 2017	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017	Oct. 31, 2019	Jan. 01, 2019	Jun. 29, 2017
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 or (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			$ 1,070,000
Maximum market value of common stock held by non-affiliates to be remained in emerging growth company										$ 700,000
Additional non-convertible debt securities to be remained in emerging growth company			1,000,000
Accumulated deficit			(166,376)		$ (122,828)	[1]
Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Non-refundable, non-creditable upfront cash payments			$ 35,000
Payment received for license agreement		$ 5,000			5,000		$ 25,000
Additional payment to be received for license agreement	$ 5,000
Taiho Pharmaceutical Co Ltd | Taiho Agreement | Scenario Forecast
Significant Accounting Policies [Line Items]
Payment received for license agreement				$ 5,000
Additional payment to be received for license agreement				$ 5,000
ASC 606
Significant Accounting Policies [Line Items]
Accumulated deficit									$ (120,616)
Non-refundable upfront research and development fees term			5 years
ASC 606 | Taiho Pharmaceutical Co Ltd | Taiho Agreement
Significant Accounting Policies [Line Items]
Accumulated deficit					2,200
Non-refundable upfront research and development fees term			5 years
Non refundable and non creditable upfront cash payment received					30,000
Non-refundable, non-creditable upfront cash payments			$ 35,000
Payment received for license agreement							$ 25,000
Additional payment to be received for license agreement					5,000
Net reduction to deferred revenue					$ 2,200
ASC 606 | Taiho Pharmaceutical Co Ltd | Taiho Agreement | Scenario Forecast
Significant Accounting Policies [Line Items]
Remaining non refundable and non creditable cash payments receivable								$ 5,000
ASC 606 | Adjustment due to The Adoption of Topic 606
Significant Accounting Policies [Line Items]
Accumulated deficit									$ 2,212

[1] The Condensed Consolidated Balance Sheet as of December 31, 2018 has been derived from the audited financial statements as of that date.