Asset Acquisition, Clinical and Regulatory Milestones, Maximum Contingent Share Issuance

No definition available.

Name:	lyel_AssetAcquisitionClinicalAndRegulatoryMilestonesMaximumContingentShareIssuance
Namespace Prefix:	lyel_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Asset Acquisition, Clinical Milestone, Contingent Cash Payment

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionClinicalMilestoneContingentCashPayment
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Acquisition, Commercial Sales Milestones, Maximum Contingent Cash Payment

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionCommercialSalesMilestonesMaximumContingentCashPayment
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Acquisition, Contingent Consideration, Equity Interests Issued and Issuable, Entity Shares Issuable

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionContingentConsiderationEquityInterestsIssuedAndIssuableEntitySharesIssuable
Namespace Prefix:	lyel_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Asset Acquisition, Contingent Consideration, Equity Issuance

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionContingentConsiderationEquityIssuance
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Acquisition, Contingent Consideration, Possible Achieved, Issuance of Common Shares

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionContingentConsiderationPossibleAchievedIssuanceOfCommonShares
Namespace Prefix:	lyel_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Asset Acquisition, Contingent Consideration, Probable Achievement, Share-Based Compensation Expense

+ References

No definition available.

+ Details

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

- Definition

Asset Acquisition, Equity Interest Issued Or Issuable, Number Of Shares

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionEquityInterestIssuedOrIssuableNumberOfShares
Namespace Prefix:	lyel_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Asset Acquisition, Late-Stage Regulatory Milestone, Maximum Contingent Cash Payment

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionLateStageRegulatoryMilestoneMaximumContingentCashPayment
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Acquisition, Prepaid Clinical Supply, Maximum Amount

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionPrepaidClinicalSupplyMaximumAmount
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Acquisition, Share Issuance

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionShareIssuance
Namespace Prefix:	lyel_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Asset Acquisition, Tiered Royalties, Annual Net Sales, Maximum Percentage

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionTieredRoyaltiesAnnualNetSalesMaximumPercentage
Namespace Prefix:	lyel_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Asset Acquisition, Upfront Cash Payment

+ References

No definition available.

+ Details

Name:	lyel_AssetAcquisitionUpfrontCashPayment
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Payments for Asset Acquisitions, Net of Cash Acquired

+ References

No definition available.

+ Details

Name:	lyel_PaymentsForAssetAcquisitionsNetOfCashAcquired
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Prepaid Clinical Supply

+ References

No definition available.

+ Details

Name:	lyel_PrepaidClinicalSupply
Namespace Prefix:	lyel_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 350
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482573/350-20-50-4

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 805
-SubTopic 50
-Name Accounting Standards Codification
-Section 15
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480123/805-50-15-3

+ Details

Name:	us-gaap_AssetAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase (decrease) of consideration paid in advance for supplies that provide economic benefits in future periods.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (a)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

+ Details

Name:	us-gaap_IncreaseDecreaseInPrepaidSupplies
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_AssetAcquisitionAxis=lyel_ICTLicenseAcquisitionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AssetAcquisitionAxis=lyel_ImmPACTBioUSAInc.Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=lyel_InnovativeCellularTherapeuticsHoldingsLimitedAndInnovativeCellularTherapeuticsIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Asset Acquisitions and Contingent Consideration (Details) - USD ($) $ in Millions			1 Months Ended	3 Months Ended
	Nov. 06, 2025	Oct. 31, 2024	Jul. 31, 2025	Mar. 31, 2026	Dec. 31, 2025
ICT License Acquisition
Asset Acquisition [Line Items]
Increase in prepaid clinical supply				$ 1.7
Prepaid clinical supply				$ 5.8	$ 7.5
Asset acquisition, contingent consideration, possible achievement, issuance of common shares				1,100,000
Asset acquisition, contingent consideration probable achievement, share-based compensation expense					$ 19.7
ImmPACT Bio USA Inc.
Asset Acquisition [Line Items]
Asset acquisition, upfront payment		$ 41.9
Stock issued during period, shares, acquisitions (in shares)		1,875,000
Asset acquisition, contingent consideration arrangements, entity shares issuable (in shares)		625,000
Equity issued for contingent consideration			$ 5.9
Innovative Cellular Therapeutics Holdings Limited and Innovative Cellular Therapeutics, Inc \| ICT License Acquisition
Asset Acquisition [Line Items]
Prepaid clinical supply cap	$ 10.0
Asset acquisition, upfront cash payment	$ 40.0
Asset acquisition, share issuance (in shares)	1,900,000
Asset acquisition, clinical milestone, contingent cash payment	$ 30.0
Asset acquisition, late-stage regulatory milestone, maximum contingent cash payment	115.0
Asset acquisition, commercial sales milestones, maximum contingent cash payment	$ 675.0
Asset acquisition, clinical and regulatory milestones, maximum contingent share issuance	1,850,000
Asset acquisition, tiered royalties, annual net sales, maximum percentage	10.00%