Fair Value of Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments	$ 1,400	$ 28,347
Short-term investments
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments	1,400	28,347
Short-term investments | Corporate bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments		19,729
Short-term investments | Municipal bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments	1,400	8,618
Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	51,037	95,927
Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Money market funds	49,637	67,580
Fair value on a recurring basis | Short-term investments | Corporate bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments		19,729
Fair value on a recurring basis | Short-term investments | Municipal bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments	1,400	8,618
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	49,637	67,580
Quoted Prices in Active Markets for Identical Assets or Liabilities (Level 1) | Fair value on a recurring basis | Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Money market funds	49,637	67,580
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Total Assets	1,400	28,347
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Short-term investments | Corporate bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments		19,729
Significant Other Observable Inputs (Level 2) | Fair value on a recurring basis | Short-term investments | Municipal bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Short-term investments	$ 1,400	$ 8,618