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Deposits - Summary of fair value and changes in fair value of structured note liabilities (Detail) - CAD ($) $ in Millions | 12 Months Ended
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Oct. 31, 2025 | Oct. 31, 2024
Disclosure of Fair Value Measurements of Deposits designated at FVTPL [Abstract] |  |
Fair value | $ 56,162 | $ 54,108
Notional amount due at contractual maturity | 57,743 | 56,300
Difference between fair value and amount due at contractual maturity | (1,581) | (2,192)
Change in fair value - gains (losses) recorded in the Consolidated Statement of Income | (1,313) | (4,815)
Change in fair value – (losses) due to own credit risk recorded in OCI (before tax | (365) | (841)
Cumulative change in fair value – gains (losses) due to own credit risk recognized in AOCI (before tax | $ (341) | $ 24

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- Definition Change in fair value - gains (losses) recorded in the Consolidated Statement of Income. + References No definition available. + Details
Name: | bmo_ChangeInFairValueGainsLossesRecordedInTheConsolidatedStatementOfIncome
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Change in fair value of financial liability attributable to change in credit risk of liability. + References No definition available. + Details
Name: | bmo_ChangeInFairValueOfFinancialLiabilityAttributableToChangeInCreditRiskOfLiability
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Cumulative change in deposit fair value due to own credit risk recognized in accumulated other comprehensive income before tax. + References No definition available. + Details
Name: | bmo_CumulativeChangeInDepositFairValueDueToOwnCreditRiskRecognizedInAccumulatedOtherComprehensiveIncomeBeforeTax
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Deposits at fair value. + References No definition available. + Details
Name: | bmo_DepositsAtFairValue
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Difference between fair value of deposit and amount due at contractual maturity. + References No definition available. + Details
Name: | bmo_DifferenceBetweenFairValueOfDepositAndAmountDueAtContractualMaturity
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- References No definition available. + Details
Name: | bmo_DisclosureOfFairValueMeasurementsOfDepositsDesignatedAtFvtplAbstract
Namespace Prefix: | bmo_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Notional amount of deposits at contractual maturity. + References No definition available. + Details
Name: | bmo_NotionalAmountOfDepositsatContractualMaturity
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant