Fair value measurement - Summary of carrying values and fair values of financial instruments (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Trade and other payables
FINANCIAL LIABILITIES
Carrying value	$ 115.7	$ 111.3
Fair value	115.7	111.3
Long-term debt
FINANCIAL LIABILITIES
Carrying value	489.2	714.5
Fair value	548.0	707.7
Gold stream obligation
FINANCIAL LIABILITIES
Carrying value	217.9	164.5
Fair value	217.9	164.5
New Afton
FINANCIAL LIABILITIES
Carrying value	436.1	0.0
Fair value	436.1	0.0
Gold price option contracts
FINANCIAL LIABILITIES
Carrying value	0.0	26.4
Fair value	0.0	26.4
Cash and cash equivalents
FINANCIAL ASSETS
Carrying value	186.3	83.4
Fair value	186.3	83.4
Trade and other receivables
FINANCIAL ASSETS
Carrying value	65.2	14.5
Fair value	65.2	14.5
Provisionally priced contracts
FINANCIAL ASSETS
Carrying value	8.9	1.5
Fair value	8.9	1.5
Gold and copper swap contracts
FINANCIAL ASSETS
Carrying value	(9.8)	(1.3)
Fair value	(9.8)	(1.3)
Proceeds due from income tax refunds at Mesquite
FINANCIAL ASSETS
Carrying value	12.8	9.0
Fair value	12.8	9.0
Investments
FINANCIAL ASSETS
Carrying value	46.2	0.5
Fair value	$ 46.2	$ 0.5

The amount of assets that are: (a) cash; (b) an equity instrument of another entity; (c) a contractual right: (i) to receive cash or another financial asset from another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially favourable to the entity; or (d) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to receive a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose the entity’s own equity instruments do not include puttable financial instruments classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. [Refer: Financial instruments, class [member]; Financial liabilities]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 25
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_25&doctype=Standard
-URIDate 2020-01-01

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 35H
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_35H&doctype=Standard
-URIDate 2020-01-01

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 35I
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_35I&doctype=Standard
-URIDate 2020-01-01

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 35M
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_35M&doctype=Standard
-URIDate 2020-01-01

Reference 5: http://www.xbrl.org/2003/role/exampleRef
-Name IFRS
-Number 7
-IssueDate 2020-01-01
-Paragraph 35N
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2020-en-r&anchor=para_35N&doctype=Standard
-URIDate 2020-01-01

+ Details

Name:	ifrs-full_FinancialAssets
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The fair value of financial assets. [Refer: At fair value [member]; Financial assets]

+ References

+ Details

Name:	ifrs-full_FinancialAssetsAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The amount of liabilities that are: (a) a contractual obligation: (i) to deliver cash or another financial asset to another entity; or (ii) to exchange financial assets or financial liabilities with another entity under conditions that are potentially unfavourable to the entity; or (b) a contract that will, or may be, settled in the entity’s own equity instruments and is: (i) a non-derivative for which the entity is, or may be, obliged to deliver a variable number of the entity’s own equity instruments; or (ii) a derivative that will, or may be, settled other than by the exchange of a fixed amount of cash or another financial asset for a fixed number of the entity’s own equity instruments. For this purpose, rights, options or warrants to acquire a fixed number of the entity’s own equity instruments for a fixed amount of any currency are equity instruments if the entity offers the rights, options or warrants pro rata to all of its existing owners of the same class of its own non-derivative equity instruments. Also, for those purposes the entity’s own equity instruments do not include puttable financial instruments that are classified as equity instruments in accordance with paragraphs 16A-16B of IAS 32, instruments that impose on the entity an obligation to deliver to another party a pro rata share of the net assets of the entity only on liquidation and are classified as equity instruments in accordance with paragraphs 16C-16D of IAS 32, or instruments that are contracts for the future receipt or delivery of the entity’s own equity instruments. As an exception, an instrument that meets the definition of a financial liability is classified as an equity instrument if it has all the features and meets the conditions in paragraphs 16A-16B or paragraphs 16C-16D of IAS 32. [Refer: Financial instruments, class [member]; Financial assets; Derivatives [member]]

+ References

+ Details

Name:	ifrs-full_FinancialLiabilities
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The fair value of financial liabilities. [Refer: At fair value [member]; Financial liabilities]

+ References

+ Details

Name:	ifrs-full_FinancialLiabilitiesAtFairValue
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

Name:	ngd_FinancialAssetsAbstract
Namespace Prefix:	ngd_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

Name:	ngd_FinancialLiabilitiesAbstract
Namespace Prefix:	ngd_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	ifrs-full_TypesOfFinancialLiabilitiesAxis=ngd_TradeAndOtherPayablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialLiabilitiesAxis=ngd_LongtermDebtsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialLiabilitiesAxis=ngd_GoldStreamObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialLiabilitiesAxis=ngd_FreeCashFlowInterestObligationNewAftonMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialLiabilitiesAxis=ngd_GoldPriceOptionContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_CashAndCashEquivalentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_TradeAndOtherReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_ProvisionallyPricedContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_GoldAndCopperSwapContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_ProceedsDueFromIncomeTaxRefundsAtMesquiteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfFinancialAssetsAxis=ngd_InvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: