Statements of Stockholders' Equity (Deficit) (Unaudited) - USD ($) $ in Thousands	Total	Takeda License	Common Stock	Common Stock Takeda License	Common Stock Frazier	Additional Paid-in Capital	Additional Paid-in Capital Takeda License	Accumulated Deficit
Beginning balance at Dec. 31, 2018	$ (1,286)		$ 0			$ 2		$ (1,288)
Beginning balance, shares at Dec. 31, 2018			0
Merger of entities under common control into the Company, shares			6,760,334
Vesting restrictions placed on previously issued and outstanding common stock, shares			(3,373,408)
Issuance of common stock, shares					1,491,072
Vesting of restricted shares, shares			1,054,192
Net loss	(1,251)							(1,251)
Ending balance at Mar. 31, 2019	(2,537)					2		(2,539)
Ending balance, shares at Mar. 31, 2019			5,932,190
Beginning balance at Dec. 31, 2018	(1,286)		$ 0			2		(1,288)
Beginning balance, shares at Dec. 31, 2018			0
Net loss	(157,125)
Ending balance at Sep. 30, 2019	(152,461)		$ 0			5,952		(158,413)
Ending balance, shares at Sep. 30, 2019			7,420,989
Beginning balance at Mar. 31, 2019	(2,537)					2		(2,539)
Beginning balance, shares at Mar. 31, 2019			5,932,190
Issuance of common stock		$ 5,885					$ 5,885
Issuance of common stock, shares				1,084,000
Vesting of restricted shares, shares			203,078
Stock-based compensation	29					29
Net loss	(87,762)							(87,762)
Ending balance at Jun. 30, 2019	(84,385)		$ 0			5,916		(90,301)
Ending balance, shares at Jun. 30, 2019			7,219,268
Vesting of restricted shares, shares			201,721
Stock-based compensation	36					36
Net loss	(68,112)							(68,112)
Ending balance at Sep. 30, 2019	(152,461)		$ 0			5,952		(158,413)
Ending balance, shares at Sep. 30, 2019			7,420,989
Beginning balance at Dec. 31, 2019	$ 227,955		$ 2			484,372		(256,419)
Beginning balance, shares at Dec. 31, 2019	24,728,258		24,728,258
Conversion of Lender Warrants into equity	$ 318					318
Vesting of restricted shares, shares			425,295
Stock-based compensation	563					563
Net loss	(20,141)							(20,141)
Ending balance at Mar. 31, 2020	208,695		$ 2			485,253		(276,560)
Ending balance, shares at Mar. 31, 2020			25,153,553
Beginning balance at Dec. 31, 2019	$ 227,955		$ 2			484,372		(256,419)
Beginning balance, shares at Dec. 31, 2019	24,728,258		24,728,258
Net loss	$ (75,347)
Ending balance at Sep. 30, 2020	$ 156,388		$ 3			488,151		(331,766)
Ending balance, shares at Sep. 30, 2020	25,916,093		25,916,093
Beginning balance at Mar. 31, 2020	$ 208,695		$ 2			485,253		(276,560)
Beginning balance, shares at Mar. 31, 2020			25,153,553
Vesting of restricted shares	1		$ 1
Vesting of restricted shares, shares			460,859
Stock-based compensation	828					828
Net loss	(21,101)							(21,101)
Ending balance at Jun. 30, 2020	188,423		$ 3			486,081		(297,661)
Ending balance, shares at Jun. 30, 2020			25,614,412
Vesting of restricted shares, shares			301,681
Stock-based compensation	2,070					2,070
Net loss	(34,105)							(34,105)
Ending balance at Sep. 30, 2020	$ 156,388		$ 3			$ 488,151		$ (331,766)
Ending balance, shares at Sep. 30, 2020	25,916,093		25,916,093