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Cash Equivalents and Marketable Securities - Additional Information (Detail) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract] |  |  |
Realized losses of available-for-sale securities | $ 0 | $ 0 | $ 0
Investments other-than-temporarily impaired | $ 0 | $ 0 |

X
- Definition Other than temporarily impaired investment. + References No definition available. + Details
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Namespace Prefix: | sgmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563 + Details
Name: | us-gaap_DebtSecuritiesAvailableForSaleRealizedLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration