Fair Value of Financial Assets and Liabilities - (Details) - Recurring - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Money market funds, included in cash and cash equivalents	$ 103,137	$ 55,291
Marketable securities:
US Treasury obligation		1,498
Total assets	103,137	56,789
Liabilities:
Warrant to purchase redeemable security		37
Total liabilities		37
Level 1
Assets:
Money market funds, included in cash and cash equivalents	103,137	55,291
Marketable securities:
US Treasury obligation		1,498
Total assets	$ 103,137	56,789
Level 3
Liabilities:
Warrant to purchase redeemable security		37
Total liabilities		$ 37