RELATED PARTY TRANSACTIONS (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Key management personnel of the Company
Employee benefit expenses	$ 2,641	$ 1,158
Share-based payments	2,475	3,435
Directors of the Company
Board fees	829	643
Share-based payments	$ 1,247	$ 2,333

The amount of remuneration paid or payable to the entity's directors.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2022-03-24
-Paragraph 112
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2022-en-r&anchor=para_112_c&doctype=Standard
-URIDate 2022-03-24

+ Details

Name:	ifrs-full_DirectorsRemunerationExpense
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of compensation to key management personnel in the form of share-based payments. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2022-03-24
-Paragraph 17
-Subparagraph e
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=24&code=ifrs-tx-2022-en-r&anchor=para_17_e&doctype=Standard
-URIDate 2022-03-24

+ Details

Name:	ifrs-full_KeyManagementPersonnelCompensationSharebasedPayment
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of compensation to key management personnel in the form of short-term employee benefits. [Refer: Key management personnel of entity or parent [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 24
-IssueDate 2022-03-24
-Paragraph 17
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=24&code=ifrs-tx-2022-en-r&anchor=para_17_a&doctype=Standard
-URIDate 2022-03-24

+ Details

Name:	ifrs-full_KeyManagementPersonnelCompensationShorttermEmployeeBenefits
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The amount of compensation to directors in the form of share-based payments.

+ References

No definition available.

+ Details

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

- References

No definition available.

+ Details

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