| Cash, Cash Equivalents and Investments [Table Text Block] |
| | | | | | | | | | | | | | | | | | | | Cash and cash equivalents: | | | | | | | | | | | | | | | | | | Cash | | $ | 3,018 | | | $ | — | | | $ | — | | | $ | 3,018 | | | Money market funds | | | 286 | | | | — | | | | — | | | | 286 | | | Commercial paper | | | 61,767 | | | | — | | | | — | | | | 61,767 | | | Total cash and cash equivalents | | | 65,071 | | | | — | | | | — | | | | 65,071 | | | | | | | | | | | | | | | | | | | | | Short-term investments: | | | | | | | | | | | | | | | | | | Commercial paper | | $ | 26,475 | | | $ | — | | | $ | — | | | $ | 26,475 | | | Total cash, cash equivalents and investments | | $ | 91,546 | | | $ | — | | | $ | — | | | $ | 91,546 | | | | | | | | | | | | | | | | | | | | | | Cash and cash equivalents: | | | | | | | | | | | | | | | | | | Cash | | $ | 2,037 | | | $ | — | | | $ | — | | | $ | 2,037 | | | Money market funds | | | 1,436 | | | | — | | | | — | | | | 1,436 | | | U.S. government agency securities | | | 10,181 | | | | — | | | | — | | | | 10,181 | | | Commercial paper | | | 74,321 | | | | — | | | | — | | | | 74,321 | | | Total cash and cash equivalents | | | 87,975 | | | | — | | | | — | | | | 87,975 | | | | | | | | | | | | | | | | | | | | | Short-term investments: | | | | | | | | | | | | | | | | | | U.S. government agency securities | | $ | 1,497 | | | $ | — | | | $ | — | | | $ | 1,497 | | | Commercial paper | | | 16,243 | | | | — | | | | — | | | | 16,243 | | | Total marketable securities and commercial paper | | | 17,740 | | | | — | | | | — | | | | 17,740 | | | | | | | | | | | | | | | | | | | | | Total cash, cash equivalents and investments | | $ | 105,715 | | | $ | — | | | $ | — | | | $ | 105,715 | |
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| Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] |
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Money market funds | | $ | 286 | | | $ | 286 | | | $ | — | | | $ | — | | | Commercial paper | | | 88,242 | | | | — | | | | 88,242 | | | | — | | | | | | | | | | | | | | | | | | | | | Total assets measured at fair value | | $ | 88,528 | | | $ | 286 | | | $ | 88,242 | | | $ | — | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Contingent put option liability | | $ | 657 | | | $ | — | | | $ | — | | | $ | 657 | | | | | | | | | | | | | | | | | | | | | Total liabilities measured at fair value | | $ | 657 | | | $ | — | | | $ | — | | | $ | 657 | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Money market funds | | $ | 1,436 | | | $ | 1,436 | | | $ | — | | | $ | — | | | U.S. government agency securities | | | 11,678 | | | | — | | | | 11,678 | | | | — | | | Commercial paper | | | 90,564 | | | | — | | | | 90,564 | | | | — | | | | | | | | | | | | | | | | | | | | | Total assets measured at fair value | | $ | 103,678 | | | $ | 1,436 | | | $ | 102,242 | | | $ | — | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | Contingent put option liability | | $ | 121 | | | $ | — | | | $ | — | | | $ | 121 | | | | | | | | | | | | | | | | | | | | | Total liabilities measured at fair value | | $ | 121 | | | $ | — | | | $ | — | | | $ | 121 | |
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| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] |
| | | Three Months Ended June 30, 201 9 | | | Six Months Ended June 30, 201 9 | | | Fair value—beginning of period | | $ | 98 | | | $ | 121 | | | Fair value of contingent put option associated with the Loan Agreement | | | 657 | | | | 657 | | | Change in fair value of contingent put option associated with the Prior Agreement | | | (98 | ) | | | (121 | ) | | Fair value—end of period | | $ | 657 | | | $ | 657 | | | | | Three Months Ended June 30, 201 8 | | | Six Months Ended June 30, 201 8 | | | Fair value—beginning of period | | $ | 186 | | | $ | 207 | | | Change in fair value of contingent put option associated with the Prior Agreement | | | (20 | ) | | | (41 | ) | | Fair value—end of period | | $ | 166 | | | $ | 166 | |
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