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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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