Fair Value of Financial Assets and Liabilities - (Details) - Recurring - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets:
Money market funds, included in cash and cash equivalents	$ 79,259	$ 34,896
Marketable securities:
U.S. Treasury obligations		121,140
Total assets	157,467	156,036
Cash and cash equivalents
Marketable securities:
U.S. Treasury obligations	12,994
Marketable securities.
Marketable securities:
U.S. Treasury obligations	65,214
Level 1
Assets:
Money market funds, included in cash and cash equivalents	79,259	34,896
Marketable securities:
U.S. Treasury obligations		121,140
Total assets	157,467	$ 156,036
Level 1 | Cash and cash equivalents
Marketable securities:
U.S. Treasury obligations	12,994
Level 1 | Marketable securities.
Marketable securities:
U.S. Treasury obligations	$ 65,214