Fair Value Measurements (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Assets:
Assets, fair value	$ 135,939	$ 187,817
Liabilities:
Liabilities, fair value	680	1,376
Derivative liability for Exit Fee
Liabilities:
Liabilities, fair value	680	1,376
Money market funds
Assets:
Assets, fair value	68,326	88,151
Commercial paper
Assets:
Assets, fair value	51,172	62,729
Corporate bonds
Assets:
Assets, fair value	11,101	26,658
Asset-backed securities
Assets:
Assets, fair value	5,340
U.S. government-sponsored agency bonds
Assets:
Assets, fair value		9,279
U.S. treasury notes
Assets:
Assets, fair value		1,000
Level 1
Assets:
Assets, fair value	68,326	88,151
Liabilities:
Liabilities, fair value	0	0
Level 1 | Derivative liability for Exit Fee
Liabilities:
Liabilities, fair value	0	0
Level 1 | Money market funds
Assets:
Assets, fair value	68,326	88,151
Level 1 | Commercial paper
Assets:
Assets, fair value	0	0
Level 1 | Corporate bonds
Assets:
Assets, fair value	0	0
Level 1 | Asset-backed securities
Assets:
Assets, fair value	0
Level 1 | U.S. government-sponsored agency bonds
Assets:
Assets, fair value		0
Level 1 | U.S. treasury notes
Assets:
Assets, fair value		0
Level 2
Assets:
Assets, fair value	67,613	99,666
Liabilities:
Liabilities, fair value	0	0
Level 2 | Derivative liability for Exit Fee
Liabilities:
Liabilities, fair value	0	0
Level 2 | Money market funds
Assets:
Assets, fair value	0	0
Level 2 | Commercial paper
Assets:
Assets, fair value	51,172	62,729
Level 2 | Corporate bonds
Assets:
Assets, fair value	11,101	26,658
Level 2 | Asset-backed securities
Assets:
Assets, fair value	5,340
Level 2 | U.S. government-sponsored agency bonds
Assets:
Assets, fair value		9,279
Level 2 | U.S. treasury notes
Assets:
Assets, fair value		1,000
Level 3
Assets:
Assets, fair value	0	0
Liabilities:
Liabilities, fair value	680	1,376
Level 3 | Derivative liability for Exit Fee
Liabilities:
Liabilities, fair value	680	1,376
Level 3 | Money market funds
Assets:
Assets, fair value	0	0
Level 3 | Commercial paper
Assets:
Assets, fair value	0	0
Level 3 | Corporate bonds
Assets:
Assets, fair value	0	0
Level 3 | Asset-backed securities
Assets:
Assets, fair value	$ 0
Level 3 | U.S. government-sponsored agency bonds
Assets:
Assets, fair value		0
Level 3 | U.S. treasury notes
Assets:
Assets, fair value		$ 0