Fair Value Measurements (Details) - USD ($) $ in Thousands | Jun. 30, 2016 | Dec. 31, 2015 |
U. S. treasury debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | $ 3,513 | $ 4,774 |
U.S. corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 7,531 | 12,646 |
U.S. corporate debt securities | Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 11,044 | 17,420 |
Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | 34,667 | 53,365 |
Money market funds | Cash and cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | 34,667 | 53,365 |
Level 3 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Long-term debt fair value | 73 | |
Recurring | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets measured at fair value | 45,711 | 70,785 |
Recurring | Cash and cash equivalents | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | 34,667 | 53,365 |
Recurring | Marketable securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 11,044 | 17,420 |
Recurring | U. S. treasury debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 3,513 | 4,774 |
Recurring | U.S. corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 7,531 | 12,646 |
Recurring | Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | 34,667 | 53,365 |
Recurring | Level 1 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets measured at fair value | 34,667 | 53,365 |
Recurring | Level 1 | U. S. treasury debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 0 | 0 |
Recurring | Level 1 | U.S. corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 0 | 0 |
Recurring | Level 1 | Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | 34,667 | 53,365 |
Recurring | Level 2 | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Total assets measured at fair value | 11,044 | 17,420 |
Recurring | Level 2 | U. S. treasury debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 3,513 | 4,774 |
Recurring | Level 2 | U.S. corporate debt securities | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Marketable securities - domestic debt securities | 7,531 | 12,646 |
Recurring | Level 2 | Money market funds | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash and cash equivalents - money market funds | $ 0 | $ 0 |