Fair Value Measurements - Assets measured at fair value on a recurring basis (Details) - Fair value on recurring basis - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	$ 48,093	$ 50,790
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Corporate debt securities	996	951
Fair Value Measurement, Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds (included in cash and cash equivalents)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds (included in cash and cash equivalents)	47,097	49,839
Fair Value Measurement, Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value
Fair Value Measurement, Significant Other Observable Inputs (Level 2) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Corporate debt securities
Fair Value Measurement, Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value
Fair Value Measurement, Significant Unobservable Inputs (Level 3) | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Corporate debt securities