Fair Value Measurements (Details) - Recurring basis - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Total assets	$ 35,497	$ 58,356
U.S. Treasury securities
Assets
Investments	24,902	24,825
Money market funds
Assets
Cash and cash equivalents, fair value	10,595	33,531
Level 1
Assets
Total assets	10,595	33,531
Level 1 | Money market funds
Assets
Cash and cash equivalents, fair value	10,595	33,531
Level 2
Assets
Total assets	24,902	24,825
Level 2 | U.S. Treasury securities
Assets
Investments	$ 24,902	$ 24,825