Derivative Instruments [Abstract]

No definition available.

Name:	bfb_DerivativeInstrumentsAbstract
Namespace Prefix:	bfb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Non-Derivative Hedging Instruments [Abstract]

+ References

No definition available.

+ Details

Name:	bfb_NonDerivativeHedgingInstrumentsAbstract
Namespace Prefix:	bfb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Nonderivative, Gain (Loss) on Nonderivative, Net

+ References

No definition available.

+ Details

Name:	bfb_NonderivativeGainLossonNonderivativeNet
Namespace Prefix:	bfb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Nonderivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net

+ References

No definition available.

+ Details

Name:	bfb_NonderivativeInstrumentsGainLossRecognizedinOtherComprehensiveIncomeLossEffectivePortionNet
Namespace Prefix:	bfb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (a),(c),(d),(e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

+ Details

Name:	us-gaap_DerivativeGainLossOnDerivativeNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4D
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624177-113959

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossReclassifiedFromAccumulatedOCIIntoIncomeEffectivePortionNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

+ Details

Name:	us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_TreasuryLockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_ForeignExchangeContractMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_SalesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_NondesignatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_IncomeStatementLocationAxis=us-gaap_OtherIncomeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bfb_NonderivativeInstrumentAxis=bfb_ForeignCurrencyDenominatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments and Hedging Activities (Gain (Loss) on Derivatives Recognized in Consolidated Statement of Operations) (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Cash Flow Hedging [Member] \| Treasury Lock [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in AOCI	$ 0	$ 0	$ 8
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Currency derivatives [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in AOCI	(54)	41	22
Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Treasury Lock [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in AOCI		8
Designated as Hedging Instrument [Member] \| Net Investment Hedging [Member] \| Currency derivatives [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in AOCI	0	8	0
Sales [Member] \| Designated as Hedging Instrument [Member] \| Cash Flow Hedging [Member] \| Currency derivatives [Member]
Derivative Instruments [Abstract]
Net gain (loss) reclassified from AOCI into earnings	(11)	40	60
Sales [Member] \| Not Designated as Hedging Instrument [Member] \| Currency derivatives [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in earnings	(5)	2	1
Other Income [Member] \| Not Designated as Hedging Instrument [Member] \| Currency derivatives [Member]
Derivative Instruments [Abstract]
Net gain (loss) recognized in earnings	9	(5)	(5)
Foreign Currency Denominated Debt [Member] \| Designated as Hedging Instrument [Member] \| Net Investment Hedging [Member]
Non-Derivative Hedging Instruments [Abstract]
Net gain (loss) recognized in AOCI	(41)	2	0
Foreign Currency Denominated Debt [Member] \| Other Income [Member] \| Not Designated as Hedging Instrument [Member]
Non-Derivative Hedging Instruments [Abstract]
Net gain (loss) recognized in earnings	$ (21)	$ 3	$ 0