ASSET RETIREMENT OBLIGATION (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
ASSET RETIREMENT OBLIGATION
Opening balance	$ 1,009	$ 621
New obligations	46	383
Change in estimate	(120)
Accretion expense	17	5
Ending balance	$ 952	$ 1,009
Discount rate used (in percent)	5.26%	3.62%
Term of completion of reclamation work (in years)	28 years

Represents the discount rate used for determining fair value of rehabilitation obligation.

No definition available.

Name:	nmg_DiscountRateUsedForDeterminationOfFairValueOfRehabilitationObligation
Namespace Prefix:	nmg_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

No definition available.

+ Details

Name:	nmg_IfrsAssetRetirementObligationAccretionExpense
Namespace Prefix:	nmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

Name:	nmg_IfrsAssetRetirementObligationDisclosureAbstract
Namespace Prefix:	nmg_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

No definition available.

+ Details

Name:	nmg_IfrsAssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	nmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

+ References

No definition available.

+ Details

Name:	nmg_IfrsAssetRetirementObligationRevisionOfEstimate
Namespace Prefix:	nmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

No definition available.

+ Details

Name:	nmg_IfrsAssetRetirementObligationsNoncurrent
Namespace Prefix:	nmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period of completion of reclamation work.

+ References

No definition available.

+ Details

Name:	nmg_TermOfCompletionOfReclamationWork
Namespace Prefix:	nmg_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration