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Loans and Allowance for Credit Losses - Schedule of Loans and Allowance for Credit Losses by Geographic Region (Parenthetical) (Detail) - Other credit instruments [member] - CAD ($) $ in Millions | Oct. 31, 2025 | Oct. 31, 2024
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Related to impaired loans [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
ACL recorded in other liabilities | $ 56 | $ 78
Related to performing loans [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
ACL recorded in other liabilities | $ 633 | $ 502

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- Definition Allowance for credit losses recorded in other liabilities. + References No definition available. + Details
Name: | bmo_AllowanceForCreditLossesRecordedInOtherLiabilities
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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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_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_ClassesOfAssetsAxis=bmo_OtherCreditInstrumentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_ImpairmentOfFinancialAssetsAxis=bmo_RelatedToImpairedLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_ImpairmentOfFinancialAssetsAxis=bmo_RelatedToPerformingLoansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |