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Derivative Instruments - Derivatives Designated As Fair Value Hedging Instruments, Hedged Items and Hedge Ineffectiveness (Parenthetical) (Detail) - CAD ($) $ in Millions | Oct. 31, 2025 | Oct. 31, 2024
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Bond Futures [Member] |  |
Disclosure of information about terms and conditions of hedging instruments and how they affect future cash flows [line items] |  |
Net fair value of bonds designated as fair value hedges | $ 0 | $ 0

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- Definition Net fair value of bonds designated as fair value hedges. + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | ifrs-full_TypesOfRisksAxis=bmo_BondFuturesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |