Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Total assets	$ 24,465	$ 19,047
U.S. Treasury securities
Assets
Investments		4,998
Money market funds
Assets
Cash and cash equivalents, fair value	24,465	14,049
Level 1
Assets
Total assets	24,465	14,049
Level 1 | Money market funds
Assets
Cash and cash equivalents, fair value	24,465	14,049
Level 2
Assets
Total assets	$ 0	4,998
Level 2 | U.S. Treasury securities
Assets
Investments		$ 4,998