Summary of Significant Accounting Policies - Assets and Liabilities Measured at Fair Value on a Recurring Basis (Details) - Fair value on a recurring basis - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	$ 95,927	$ 86,627
Money market funds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	67,580	26,056
Short-term investments | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		3,974
Short-term investments | Short-term investments – certificate of deposit
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		24,575
Short-term investments | Corporate bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	19,729	5,025
Short-term investments | Municipal bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	8,618	21,186
Long-term investments
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		5,811
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
Assets, Fair Value Disclosure	67,580	26,056
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
Assets, Fair Value Disclosure	67,580	26,056
Significant Other Observable Inputs (Level 2)
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	28,347	60,571
Significant Other Observable Inputs (Level 2) | Short-term investments | Short-term investments – commercial paper
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		3,974
Significant Other Observable Inputs (Level 2) | Short-term investments | Short-term investments – certificate of deposit
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		24,575
Significant Other Observable Inputs (Level 2) | Short-term investments | Corporate bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	19,729	5,025
Significant Other Observable Inputs (Level 2) | Short-term investments | Municipal bonds
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure	$ 8,618	21,186
Significant Other Observable Inputs (Level 2) | Long-term investments
Fair Value of Assets and Liabilities Measured at Fair Value on a Recurring Basis
Assets, Fair Value Disclosure		$ 5,811