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IFRS 7 Disclosure - Market Risk - Additional Information (Detail) $ in Millions	12 Months Ended
	Oct. 31, 2025 CAD ($)
Market risk [member] | Value at risk [member]
Market Risk [Line Items]
Percentage of trading and underwriting portfolios	99.00%
Value at risk holding period	1 day
Exceptions To Trading Book Market Capitalized As Per Standardised Approach [Member]
Market Risk [Line Items]
Fair value of financial assets liabilities subject to exception designated as trading but used to hedge banking book market tests	$ 1,378
Fair value of financial assets liabilities subject to exception designated as trading but used to hedge banking book market tests, gross	$ 12,996

X

- Definition

Financial instrument designated as hedging instrument at fair value.

+ References

No definition available.

+ Details

Name:	bmo_FinancialInstrumentDesignatedAsHedgingInstrumentAtFairValue
Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Financial instrument designated as hedging instrument at fair value gross.

+ References

No definition available.

+ Details

Name:	bmo_FinancialInstrumentDesignatedAsHedgingInstrumentAtFairValueGross
Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Market Risk [Line Items]

+ References

No definition available.

+ Details

Name:	bmo_MarketRiskLineItems
Namespace Prefix:	bmo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Percentage of trading and underwriting portfolios measurement.

+ References

No definition available.

+ Details

Name:	bmo_PercentageOfTradingAndUnderwritingPortfoliosMeasurement
Namespace Prefix:	bmo_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Value at Risk Holding Period.

+ References

No definition available.

+ Details

Name:	bmo_ValueAtRiskHoldingPeriod
Namespace Prefix:	bmo_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	ifrs-full_TypesOfRisksAxis=ifrs-full_MarketRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_InternalCreditGradesAxis=bmo_ValueAtRiskMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_TypesOfRisksAxis=bmo_ExceptionsToTradingBookMarketCapitalizedAsPerStandardisedApproachMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: