Fair Value of Financial Assets and Liabilities - (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Marketable securities:
Warrant to purchase preferred stock converted into warrant to purchase common stock	7,614
Warrant purchase price	$ 3.94
Recurring
Assets:
Money market funds, included in cash and cash equivalents	$ 34,896	$ 114,593
Marketable securities:
U.S. Treasury obligations	121,140	60,576
Total assets	156,036	175,169
Recurring | Level 1
Assets:
Money market funds, included in cash and cash equivalents	34,896	114,593
Marketable securities:
U.S. Treasury obligations	121,140	60,576
Total assets	$ 156,036	$ 175,169