PROPERTY, PLANT AND EQUIPMENT- Additional Information (Details) - CAD ($) $ in Thousands	12 Months Ended
Dec. 31, 2021	Dec. 31, 2022
PROPERTY, PLANT AND EQUIPMENT
Amount of PPE net of grants received	$ 5,483	$ 4,109
Percentage of NSR bought back	1.80%
Amount of NSR bought back	$ 1,800

Amount of property, plant and equipment net of grants received.

No definition available.

Name:	nmg_PropertyPlantAndEquipmentNetOfGrantsReceived
Namespace Prefix:	nmg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant