Organization, Basis of Presentation and Summary of Significant Accounting Policies - Schedule of Reconciliation of Liabilities Measured at Fair Value (Details) - Significant Unobservable Inputs (Level 3) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Warrant Liabilities
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Issuance of warrants	$ 48,313
Change in fair value	96,272
Reclass Takeda Warrant into equity (Note 4)	(144,172)
Ending Balance	413
Convertible Promissory Notes
Fair Value Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning Balance	1,950
Issuance of convertible promissory notes	90,750	$ 1,900
Exchange of convertible promissory notes (Note 5)	(2,399)
Change in fair value	49,546	50
Conversion of May 2019 Notes into commonshares upon IPO (Note 5)	$ (139,847)
Ending Balance		$ 1,950