Organization, Basis of Presentation and Summary of Significant Accounting Policies - Additional Information (Details)			3 Months Ended			4 Months Ended	9 Months Ended		12 Months Ended
	Oct. 29, 2019 USD ($) shares	Oct. 11, 2019 shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 shares	Mar. 31, 2019 shares	May 06, 2019 shares	Sep. 30, 2019 USD ($) Segment shares	Sep. 30, 2018 shares	Dec. 31, 2018 USD ($) $ / shares shares	May 07, 2019 USD ($)	Jun. 30, 2018 shares	Mar. 31, 2018 shares	Dec. 31, 2017 shares
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Description of forward stock split							1,559.1183-for-1
Forward stock split conversion ratio									1,559.1183
Common stock, outstanding shares | shares			7,420,989				7,420,989		6,760,334
Common stock, authorized shares | shares			50,000,000				50,000,000
Fair value liabilities, level 1 to level 2 transfers, amount			$ 0				$ 0		$ 0
Fair value liabilities, level 2 to level 1 transfers, amount			0				0		0
Fair value liability, transfers into level 3							0		0
Fair value liability, transfers out of level 3							0		0
Deferred offering costs			$ 2,000,000				$ 2,000,000		0
Number of operating segment | Segment							1
Unvested Shares
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Weighted-average unvested shares | shares			4,550,428				3,342,544	0
Frazier Notes
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Fair value of convertible notes									$ 2,000,000
May 2019 Notes
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Fair value of convertible notes			$ 95,229,000				$ 95,229,000			$ 90,250,000
Takeda Warrants
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Fair value of warrants			107,000,000				107,000,000
Lender Warrants
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Fair value of warrants			400,000				400,000
Fair Value, Recurring
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Financial assets fair value disclosure			0				0
Fair Value, Nonrecurring
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Non-financial assets fair value disclosure			0				0
Non-financial liabilities fair value disclosure			$ 0				$ 0
Subsequent Event
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Description of forward stock split		2.168-for-1
Forward stock split conversion ratio		2.168
Common stock, authorized shares | shares		50,000,000
Common Stock
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Common stock shares issued | shares				1,084,000	1,491,072	2,524,852			1,000
Purchase price | $ / shares									$ 1.00
Common stock, outstanding shares | shares			7,420,989	7,219,268	5,932,190		7,420,989	2,000	0		2,000	2,000	0
Common Stock | YamadaCo
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Convertible securities converted into shares of common stock | shares									1,559.1183
IPO | Common Stock | Subsequent Event
Organization Basis of Presentation and Summary of Significant Accounting Policies [Line Items]
Common stock shares issued | shares	10,997,630
Proceeds from Initial Public Offering (IPO)	$ 191,400,000
Convertible securities converted into shares of common stock | shares	6,107,918