Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Total assets	$ 27,504	$ 58,356
U.S. Treasury securities
Assets
Investments	19,967	24,825
Money market funds
Assets
Cash and cash equivalents, fair value	7,537	33,531
Level 1
Assets
Total assets	7,537	33,531
Level 1 | Money market funds
Assets
Cash and cash equivalents, fair value	7,537	33,531
Level 2
Assets
Total assets	19,967	24,825
Level 2 | U.S. Treasury securities
Assets
Investments	$ 19,967	$ 24,825