Fair Value of Financial Instruments - Financial assets and liabilities (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Total assets	$ 94,714	$ 109,630
Liabilities
Total liabilities		380
Common stock warrants
Liabilities
Total liabilities		380
Cash and money market funds
Assets
Total assets	59,820	109,630
U.S. Treasury securities
Assets
Total assets	34,894
Level 1
Assets
Total assets	94,714	109,630
Level 1 | Cash and money market funds
Assets
Total assets	59,820	109,630
Level 1 | U.S. Treasury securities
Assets
Total assets	$ 34,894
Level 3
Liabilities
Total liabilities		380
Level 3 | Common stock warrants
Liabilities
Total liabilities		$ 380