Fair Value of Financial Instruments - Financial assets and liabilities (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Total assets	$ 164,996	$ 178,231
Cash, cash equivalents and money market funds
Assets
Total assets	85,176	152,770
Certificates of deposit
Assets
Total assets	2,644	1,950
Commercial paper
Assets
Total assets	19,544
Corporate debt securities
Assets
Total assets	16,400	3,459
U.S. Treasury securities
Assets
Total assets	31,217	20,052
U.S. Agency bonds
Assets
Total assets	10,015
Level 1
Assets
Total assets	119,037	174,772
Level 1 | Cash, cash equivalents and money market funds
Assets
Total assets	85,176	152,770
Level 1 | Certificates of deposit
Assets
Total assets	2,644	1,950
Level 1 | U.S. Treasury securities
Assets
Total assets	31,217	20,052
Level 2
Assets
Total assets	45,959	3,459
Level 2 | Commercial paper
Assets
Total assets	19,544
Level 2 | Corporate debt securities
Assets
Total assets	16,400	$ 3,459
Level 2 | U.S. Agency bonds
Assets
Total assets	$ 10,015