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Nature of the Business and Basis of Presentation - Liquidity (Details) - USD ($) | Jun. 11, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Jun. 30, 2019 | Dec. 31, 2018
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |  |  |  |
Accumulated deficit |  | $ 379,454,000 | $ 335,753,000 |  |
Cash and Cash Equivalents [Line Items] |  |  |  |  |
Cash, cash equivalents, and restricted cash |  | 61,387,000 | $ 91,551,000 | $ 30,479,000 | $ 37,682,000
Restricted cash and cash equivalents |  | 1,700,000 |  |  |
Collaborative Arrangement |  |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |  |
Upfront cash payment | $ 75,000,000 |  |  |  |
SOBI Purchase Agreement |  |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |  |
Sale common stock | $ 25,000,000 |  |  |  |
Russian subsidiary | Unrestricted cash |  |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |  |
Cash, cash equivalents, and restricted cash |  | $ 300,000 |  |  |

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