Fair Value Measurements - Assets measured at fair value on a recurring basis (Details) - Fair value on recurring basis - USD ($) $ in Thousands | Jun. 30, 2015 | Dec. 31, 2014 |
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 Other Observable Inputs (Level 2) | 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) | | |
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 | $ 35,167 | $ 35,767 |
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 | 856 | 1,428 |
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) | $ 34,311 | $ 34,339 |
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 | | |
Fair Value Measurement, Significant Unobservable Inputs (Level 3) | 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) | | |