Summary of Significant Accounting Policies - Assets and Liabilities Measured on a Recurring Basis (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
U.S. treasury securities (included in short-term investments)	$ 29,900
Level 1
Assets:
Money market funds (included in cash and cash equivalents)	32,936	$ 47,653
U.S. treasury securities (included in short-term investments)	29,875	0
Total assets	62,811	47,653
Liabilities:
Liabilities, fair value disclosure	0	0
Level 3
Assets:
Money market funds (included in cash and cash equivalents)	0	0
U.S. treasury securities (included in short-term investments)	0	0
Total assets	0	0
Liabilities:
Liabilities, fair value disclosure	$ 82	$ 82