Fair Value of Financial Assets and Liabilities - Measured on a Recurring Basis (Details) - Fair Value, Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	$ 63,637	$ 8,134
Liabilities		5,148
Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		4,947
Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		201
Level 1
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	23,638	0
Liabilities		0
Level 1 | Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 1 | Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 2
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	39,999	8,134
Liabilities		0
Level 2 | Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 2 | Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		0
Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Cash equivalents—money market funds	$ 0	0
Liabilities		5,148
Level 3 | Preferred stock warrant liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		4,947
Level 3 | Derivative Liability
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Liabilities		$ 201