| Cash, Cash Equivalents and Investments [Table Text Block] |
| | | As of September 30, 2020 | |
| | | Amortized Cost | | | Gross Unrealized Gains | | | Gross Unrealized Losses | | | Fair Value | |
| Cash and cash equivalents: | | | | | | | | | | | | | | | | |
| Cash | | $ | 1,357 | | | $ | — | | | $ | — | | | $ | 1,357 | |
| Money market funds | | | 1,415 | | | | — | | | | — | | | | 1,415 | |
| U.S. government agency securities | | | 5,002 | | | | — | | | | — | | | | 5,002 | |
| Commercial paper | | | 12,199 | | | | — | | | | — | | | | 12,199 | |
| Total cash and cash equivalents | | | 19,973 | | | | — | | | | — | | | | 19,973 | |
| | | | | | | | | | | | | | | | | |
| Short-term investments: | | | | | | | | | | | | | | | | |
| U.S. government agency securities | | | 13,853 | | | | — | | | | — | | | | 13,853 | |
| Commercial paper | | | 9,197 | | | | — | | | | — | | | | 9,197 | |
| Total short-term investments | | | 23,050 | | | | — | | | | — | | | | 23,050 | |
| Total cash, cash equivalents and short-term investments | | $ | 43,023 | | | $ | — | | | $ | — | | | $ | 43,023 | |
| | | As of December 31, 2019 | |
| | | Amortized Cost | | | Gross Unrealized Gains | | | Gross Unrealized Losses | | | Fair Value | |
| Cash and cash equivalents: | | | | | | | | | | | | | | | | |
| Cash | | $ | 1,957 | | | $ | — | | | $ | — | | | $ | 1,957 | |
| Money market funds | | | 598 | | | | — | | | | — | | | | 598 | |
| Commercial paper | | | 12,129 | | | | — | | | | — | | | | 12,129 | |
| Total cash and cash equivalents | | | 14,684 | | | | — | | | | — | | | | 14,684 | |
| | | | | | | | | | | | | | | | | |
| Short-term investments: | | | | | | | | | | | | | | | | |
| U.S. government agency securities | | | 14,268 | | | | — | | | | — | | | | 14,268 | |
| Commercial paper | | | 27,131 | | | | — | | | | — | | | | 27,131 | |
| Corporate debt securities | | | 10,054 | | | | — | | | | — | | | | 10,054 | |
| Total short-term investments | | | 51,453 | | | | — | | | | — | | | | 51,453 | |
| Total cash, cash equivalents and short-term investments | | $ | 66,137 | | | $ | — | | | $ | — | | | $ | 66,137 | |
|
| Fair Value Measurements, Recurring and Nonrecurring [Table Text Block] |
| | | As of September 30, 2020 | |
| | | Fair Value | | | Level I | | | Level II | | | Level III | |
| Assets | | | | | | | | | | | | | | | | |
| Money market funds | | $ | 1,415 | | | $ | 1,415 | | | $ | — | | | $ | — | |
| U.S. government agency securities | | | 18,855 | | | | — | | | | 18,855 | | | | — | |
| Commercial paper | | | 21,396 | | | | — | | | | 21,396 | | | | — | |
| Total assets measured at fair value | | $ | 41,666 | | | $ | 1,415 | | | $ | 40,251 | | | $ | — | |
| | | | | | | | | | | | | | | | | |
| Liabilities | | | | | | | | | | | | | | | | |
| Contingent put option liability | | $ | 506 | | | $ | — | | | $ | — | | | $ | 506 | |
| Total liabilities measured at fair value | | $ | 506 | | | $ | — | | | $ | — | | | $ | 506 | |
| | | As of December 31, 2019 | |
| | | Fair Value | | | Level I | | | Level II | | | Level III | |
| Assets | | | | | | | | | | | | | | | | |
| Money market funds | | $ | 598 | | | $ | 598 | | | $ | — | | | $ | — | |
| U.S. government agency securities | | | 14,268 | | | | — | | | | 14,268 | | | | — | |
| Commercial paper | | | 39,260 | | | | — | | | | 39,260 | | | | — | |
| Corporate debt securities | | | 10,054 | | | | — | | | | 10,054 | | | | — | |
| Total assets measured at fair value | | $ | 64,180 | | | $ | 598 | | | $ | 63,582 | | | $ | — | |
| | | | | | | | | | | | | | | | | |
| Liabilities | | | | | | | | | | | | | | | | |
| Contingent put option liability | | $ | 437 | | | $ | — | | | $ | — | | | $ | 437 | |
| Total liabilities measured at fair value | | $ | 437 | | | $ | — | | | $ | — | | | $ | 437 | |
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| Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] |
| | | Three Months Ended September 30, 2020 | | | Nine Months Ended September 30, 2020 | |
| Fair value—beginning of period | | $ | 591 | | | $ | 437 | |
| Fair value of contingent put option associated with the Loan Agreement | | | (85 | ) | | | 69 | |
| Fair value—end of period | | $ | 506 | | | $ | 506 | |
| | | Three Months Ended September 30, 2019 | | | Nine Months Ended September 30, 2019 | |
| Fair value—beginning of period | | $ | 657 | | | $ | 121 | |
| Fair value of contingent put option associated with the Loan Agreement | | | (143 | ) | | | 514 | |
| Change in fair value of contingent put option associated with the Prior Agreement | | | — | | | | (121 | ) |
| Fair value—end of period | | $ | 514 | | | $ | 514 | |
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