EQUITY - SHARE-BASED PAYMENTS - RSUs and PSUs (Details)	12 Months Ended
Dec. 31, 2025 EquityInstruments tranche $ / shares
Weighted average share price
Number of tranches \| tranche	2
RSU
Number of RSUs
Granted	197,342
Ending balance	197,342
Weighted average share price
Granted \| $ / shares	$ 3.42
Ending balance \| $ / shares	$ 3.42
RSU \| Key management personnel
Number of RSUs
Granted	197,342
Weighted average share price
Number of tranches \| tranche	3
PSU
Number of RSUs
Granted	394,658
Ending balance	394,658
Weighted average share price
Granted \| $ / shares	$ 3.42
Ending balance \| $ / shares	$ 3.42
PSU \| Key management personnel
Number of RSUs
Granted	394,658

The number of other equity instruments (ie other than share options) granted in a share-based payment arrangement.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IFRS
-Number 2
-IssueDate 2025-01-01
-Paragraph 45
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2025-en-r&anchor=para_45&doctype=Standard
-URIDate 2025-03-27

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 2
-IssueDate 2025-01-01
-Paragraph 47
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=2&code=ifrs-tx-2025-en-r&anchor=para_47_b&doctype=Standard
-URIDate 2025-03-27

+ Details

Name:	ifrs-full_NumberOfInstrumentsOtherEquityInstrumentsGranted
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement.

+ References

+ Details

Name:	ifrs-full_NumberOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average exercise price of other equity instruments (ie other than share options) granted in a share-based payment arrangement. [Refer: Weighted average [member]]

+ References

+ Details

Name:	ifrs-full_WeightedAverageExercisePriceOfOtherEquityInstrumentsGrantedInSharebasedPaymentArrangement2019
Namespace Prefix:	ifrs-full_
Data Type:	dtr-2022:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The weighted average exercise price of other equity instruments (ie other than share options) outstanding in a share-based payment arrangement. [Refer: Weighted average [member]]

+ References

+ Details

Name:	ifrs-full_WeightedAverageExercisePriceOfOtherEquityInstrumentsOutstandingInSharebasedPaymentArrangement2019
Namespace Prefix:	ifrs-full_
Data Type:	dtr-2022:perShareItemType
Balance Type:	na
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Represents the number of tranches for award vesting in share based payment arrangement.

+ References

No definition available.

+ Details

Name:	nmg_ShareBasedPaymentArrangementAwardVestingNumberOfTranches
Namespace Prefix:	nmg_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=ifrs-full_RestrictedShareUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	nmg_IfrsTitleOfIndividualAxis=nmg_KeyManagementPersonnelMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfSharebasedPaymentArrangementsAxis=nmg_PerformanceShareUnitsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: