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LEASES - Additional Information (Details)	3 Months Ended
	Mar. 31, 2025 USD ($)
Leasehold land
Leases [Line Items]
Lessee, lease terms	50 years
Ongoing lease payments	$ 0
Bottom of Range
Leases [Line Items]
Lessee, lease terms	3 years
Top of Range
Leases [Line Items]
Lessee, lease terms	29 years

X

- Definition

Leases line items.

+ References

No definition available.

+ Details

Name:	legn_LeasesLineItems
Namespace Prefix:	legn_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Lessee leasing arrangements leases term.

+ References

No definition available.

+ Details

Name:	legn_LesseeLeasingArrangementsLeasesTerm
Namespace Prefix:	legn_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Ongoing lease payments under the land lease.

+ References

No definition available.

+ Details

Name:	legn_OngoingLeasePaymentsUnderTheLandLease
Namespace Prefix:	legn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis=ifrs-full_LandMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: