Fair Value of Financial Instruments - Financial assets and liabilities (Details) - Recurring basis - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Assets
Total assets	$ 178,231	$ 109,630
Liabilities
Total liabilities		380
Common stock warrants
Liabilities
Total liabilities		380
Cash, cash equivalents and money market funds
Assets
Total assets	152,770	109,630
Certificates of Deposit
Assets
Total assets	1,950
Corporate Debt Securities
Assets
Total assets	3,459
U.S. Treasury securities
Assets
Total assets	20,052
Level 1
Assets
Total assets	174,772	109,630
Level 1 | Cash, cash equivalents and money market funds
Assets
Total assets	152,770	109,630
Level 1 | Certificates of Deposit
Assets
Total assets	1,950
Level 1 | U.S. Treasury securities
Assets
Total assets	20,052
Level 2
Assets
Total assets	3,459
Level 2 | Corporate Debt Securities
Assets
Total assets	$ 3,459
Level 3
Liabilities
Total liabilities		380
Level 3 | Common stock warrants
Liabilities
Total liabilities		$ 380