Fair Value Measurement - Marketable Securities Measured at Fair Value on Recurring Basis (Details) - Recurring - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Marketable securities
Marketable securities	$ 88,186	$ 119,961
Money market funds
Marketable securities
Marketable securities	3,068	15,223
U.S. treasury securities
Marketable securities
Marketable securities	67,083	66,489
Corporate bonds
Marketable securities
Marketable securities	10,375	19,720
Commercial paper
Marketable securities
Marketable securities	7,660	18,529
Level 1
Marketable securities
Marketable securities	70,151	81,712
Level 1 | Money market funds
Marketable securities
Marketable securities	3,068	15,223
Level 1 | U.S. treasury securities
Marketable securities
Marketable securities	67,083	66,489
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	18,035	38,249
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	10,375	19,720
Level 2 | Commercial paper
Marketable securities
Marketable securities	7,660	18,529
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