MATERIAL ACCOUNTING POLICIES - Contingent payments (Details) $ in Thousands	Dec. 31, 2024 CAD ($)
MATERIAL ACCOUNTING POLICIES
Liability recognized	$ 0

The amount of contingent liabilities recognised in asset acquisition.

No definition available.

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