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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
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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