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IFRS 7 Disclosure - Market Risk - Additional Information (Detail) $ in Millions | 12 Months Ended
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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: |