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Merger with Mirna Therapeutics - Additional Information (Detail) $ in Millions | Aug. 28, 2017 USD ($) Director shares | Dec. 31, 2018 shares | Dec. 31, 2017 shares
--- | --- | --- | ---
Business Acquisition [Line Items] |  |  |
Merger completion date | Aug. 28, 2017 |  |
Number of members on Board of Directors | Director | 7 |  |
Exchange ratio of common stock | 0.5532 |  |
Common stock outstanding | shares |  | 25,401,479 | 16,272,617
Synlogic |  |  |
Business Acquisition [Line Items] |  |  |
Number of members on Board of Directors | Director | 5 |  |
Mirna therapeutics |  |  |
Business Acquisition [Line Items] |  |  |
Common stock outstanding | shares | 20,900,000 |  |
Market capitalization | $ | $ 35.0 |  |
Fair value, net asset (Liability) | $ | $ 42.6 |  |
Mirna therapeutics | Common Stock | Synlogic |  |  |
Business Acquisition [Line Items] |  |  |
Share ownership following merger, percent | 83.00% |  |

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