Share capital - Stock options excluded from the calculation of diluted loss per share (Details) - shares shares in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Disclosure of classes of share capital [abstract]
Stock options excluded from the calculation of diluted earnings per share	4.8	5.6

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

No definition available.

Name:	ngd_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	ngd_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration