Mining interests - Additional information (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($)
COST
Mining Interests
Refundable tax credits received	$ 2.0

It represents the amounts transferred out of construction in progress to inventories upon reaching commercial production.

No definition available.

Name:	ngd_IncreaseDecreaseThroughTransfersFromConstructionInProgressInventories
Namespace Prefix:	ngd_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration