Fair Value Measurement - Marketable Securities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Marketable securities
Marketable securities	$ 128,314	$ 88,186
Money market funds
Marketable securities
Marketable securities	52,290	3,068
U.S. treasury securities
Marketable securities
Marketable securities	57,018	67,083
Corporate bonds
Marketable securities
Marketable securities	8,614	10,375
Commercial paper
Marketable securities
Marketable securities	10,392	7,660
Level 1
Marketable securities
Marketable securities	109,308	70,151
Level 1 | Money market funds
Marketable securities
Marketable securities	52,290	3,068
Level 1 | U.S. treasury securities
Marketable securities
Marketable securities	57,018	67,083
Level 1 | Corporate bonds
Marketable securities
Marketable securities	0	0
Level 1 | Commercial paper
Marketable securities
Marketable securities	0	0
Level 2
Marketable securities
Marketable securities	19,006	18,035
Level 2 | Money market funds
Marketable securities
Marketable securities	0	0
Level 2 | U.S. treasury securities
Marketable securities
Marketable securities	0	0
Level 2 | Corporate bonds
Marketable securities
Marketable securities	8,614	10,375
Level 2 | Commercial paper
Marketable securities
Marketable securities	10,392	7,660
Level 3
Marketable securities
Marketable securities	0	0
Level 3 | Money market funds
Marketable securities
Marketable securities	0	0
Level 3 | U.S. treasury securities
Marketable securities
Marketable securities	0	0
Level 3 | Corporate bonds
Marketable securities
Marketable securities	0	0
Level 3 | Commercial paper
Marketable securities
Marketable securities	$ 0	$ 0