Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	$ 2,774
Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	42,793	$ 71,431
Total Liabilities	49,677
Fair value on a recurring basis | Warrant liability
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Liabilities	3,241
Issuance of redeemable preferred stock and warrants	2,643
Fair value on a recurring basis | Future tranche right liability
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Liabilities	46,436
Issuance of redeemable preferred stock and warrants	35,472
Fair value on a recurring basis | Cash
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	250	8,446
Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	39,769	61,177
Fair value on a recurring basis | Other cash equivalents – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments		1,808
Fair value on a recurring basis | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	2,774
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	40,019	69,623
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Cash
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	250	8,446
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Cash and Money market funds	39,769	61,177
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	2,774	1,808
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Other cash equivalents – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments		$ 1,808
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Other cash equivalents and Short-term investments	2,774
Significant Other Unobservable Inputs (Level 3) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Liabilities	49,677
Significant Other Unobservable Inputs (Level 3) | Fair value on a recurring basis | Warrant liability
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Liabilities	3,241
Significant Other Unobservable Inputs (Level 3) | Fair value on a recurring basis | Future tranche right liability
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Liabilities	$ 46,436