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FAIR VALUE MEASUREMENTS - Summary of Estimated Fair Value of Free Shares Valuation Assumptions (Detail) | Sep. 30, 2020 $ / shares | Sep. 30, 2020 € / shares | Dec. 31, 2019 $ / shares | Dec. 31, 2019 € / shares | Oct. 01, 2018 $ / shares
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Stock Price |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Sangamo Stock Price (USD) (in dollars per share) | $ 10.31 |  | $ 8.68 |  |
Exchange Rate |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
EUR / USD exchange rate | 0.84 | 0.84 | 0.91 | 0.91 |
Stock Price Volatility Estimate |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Volatility Estimate | 65.7 | 65.7 | 72.5 | 72.5 |
Price Volatility |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Volatility Estimate | 6.4 | 6.4 | 6.6 | 6.6 |
Sangamo France |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Sangamo France Stock Price (EUR) (in euros per share) |  |  |  |  | $ 2.99
Sangamo France | Stock Price |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Sangamo France Stock Price (EUR) (in euros per share) | € / shares |  | € 2.55 |  | € 2.14 |
Sangamo France | Stock Price Correlation |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Estimated correlation Sangamo and Sangamo France stock prices | 100.0 | 100.0 | 100.0 | 100.0 |
Sangamo France | Stock Price Volatility Estimate |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |
Volatility Estimate | 65.7 | 65.7 | 72.5 | 72.5 |

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