Fair Value Measurements - Assets and Liabilities at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	$ 17,746	$ 66,183
Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Money market funds	17,746	66,183
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1)
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	17,746	66,183
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Money market funds	$ 17,746	$ 66,183