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Condensed Consolidated Statement of Convertible Preferred Stock and Stockholders' Equity (Deficit) (Parenthetical) (Unaudited) $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($)
Convertible Preferred Stock
Net issuance cost	$ 533

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- Definition

Temporary equity issuance costs.

+ References

No definition available.

+ Details

Name:	lyel_TemporaryEquityIssuanceCosts
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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- Details

Name:	us-gaap_StatementClassOfStockAxis=us-gaap_ConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: