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IFRS 7 Disclosure - Credit and Counterparty Risk - Schedule of Total Credit Risk Exposures (Parenthetical) (Detail) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Oct. 31, 2025 | Oct. 31, 2024
Repo style transactions [member] |  |
Disclosure of credit risk exposure [line items] |  |
Credit exposure | $ 332,756 | $ 270,482
Derivative and Repo Style Transactions [member] |  |
Disclosure of credit risk exposure [line items] |  |
Credit exposure exclusions at default based on Basel III classifications | $ 8,137 | $ 7,086
Other sector [member] |  |
Disclosure of credit risk exposure [line items] |  |
Credit exposure ratio | 2.00% | 2.00%

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- Definition Credit exposure exclusions at default based on basel III classifications. + References No definition available. + Details
Name: | bmo_CreditExposureExclusionsAtDefaultBasedOnBaselIiiClassifications
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Impact of collateral on credit exposure. + References No definition available. + Details
Name: | bmo_ImpactOfCollateralOnCreditExposure
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Maximum credit exposure ratio. + References No definition available. + Details
Name: | bmo_MaximumCreditExposureRatio
Namespace Prefix: | bmo_
Data Type: | dtr-types:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table. + References No definition available. + Details
Name: | ifrs-full_DisclosureOfCreditRiskExposureLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_ClassesOfFinancialInstrumentsAxis=bmo_DerivativeAndRepoStyleTransactionsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | bmo_CreditExposureAtDefaultByIndustryAxis=bmo_OtherSectorMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |