INFORMATION DISCLOSURE ABOUT CAPITAL MANAGEMENT (Details) - CAD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023
INFORMATION DISCLOSURE ABOUT CAPITAL MANAGEMENT
Carrying amount of equity, borrowings, leases and convertible loans	$ 172,621	$ 111,899

Carrying amount of equity, borrowings, leases and convertible loans.

No definition available.

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