Derivative financial instruments - Fuel Derivatives Narrative (Details) $ in Thousands, l in Millions	12 Months Ended
Dec. 31, 2019 USD ($) l	Dec. 31, 2018 USD ($)
Disclosure of detailed information about financial instruments [line items]
Gains on derivative instruments (Note 17)	$ 1,580	$ 412
Fuel Derivatives
Disclosure of detailed information about financial instruments [line items]
Unrealized gains (losses) on change in fair value measurement	(1,000)	9,000
Gains on derivative instruments (Note 17)	$ 3,000	$ 9,000
Forward contracts \| Not Designated As Hedging Instrument \| Forward – gas oil:
Disclosure of detailed information about financial instruments [line items]
Forward fuel contracts \| l	24.9
Collar Contracts \| Not Designated As Hedging Instrument \| Collars - fuel oil
Disclosure of detailed information about financial instruments [line items]
Forward fuel contracts \| l	37.4
Collar Contracts \| Not Designated As Hedging Instrument \| Collars - diesel
Disclosure of detailed information about financial instruments [line items]
Forward fuel contracts \| l	12.9
Forward And Collar Contracts [Member] \| Fuel Derivatives
Disclosure of detailed information about financial instruments [line items]
Financial assets, at fair value	$ (1,000)

Derivative, Nonmonetary Notional Amount, Volume1

No definition available.

Name:	btg_DerivativeNonmonetaryNotionalAmountVolume1
Namespace Prefix:	btg_
Data Type:	num:volumeItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2019-01-01
-Paragraph 25
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2019-en-r&anchor=para_25&doctype=Standard
-URIDate 2019-03-27

+ Details

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

- Definition

The gains (losses) resulting from change in the fair value of derivatives recognised in profit or loss. [Refer: Derivatives [member]]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 1
-IssueDate 2019-01-01
-Paragraph 85
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=1&code=ifrs-tx-2019-en-r&anchor=para_85&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_GainsLossesOnChangeInFairValueOfDerivatives
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The gains (losses) recognised in profit or loss for liabilities measured at fair value that are attributable to the change in unrealised gains or losses relating to those liabilities held at the end of the reporting period. [Refer: At fair value [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 13
-IssueDate 2019-01-01
-Paragraph 93
-Subparagraph f
-URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IFRS&num=13&code=ifrs-tx-2019-en-r&anchor=para_93_f&doctype=Standard
-URIDate 2019-03-27

+ Details

Name:	ifrs-full_GainsLossesRecognisedInProfitOrLossAttributableToChangeInUnrealisedGainsOrLossesForLiabilitiesHeldAtEndOfPeriodFairValueMeasurement
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	btg_DerivativeinstrumentAxis=btg_FuelDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	btg_HedgingDesignation1Axis=btg_NotDesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	btg_DerivativeinstrumentAxis=btg_GasOilForwardContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	btg_DerivativeinstrumentAxis=btg_FuelOilCollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	btg_DerivativeinstrumentAxis=btg_GasOilCollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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