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Derivative Instruments - Additional Information (Detail) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Oct. 31, 2025 | Oct. 31, 2024
Disclosure of detailed information about financial instruments [line items] |  |
Obligation to exercise the option | $ 146 | $ 67
Derivative instruments in a liability position | 10,894 | 9,656
Derivative instruments in a liability position, collateral posted | 9,117 | 8,882
Loan commitment derivatives with notional | 6,219 | $ 2,498
Credit risk [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
Obligation to exercise the option | $ 0 |

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- Definition Collateral posted for derivatives in liability position. + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Fair value of derivatives in liability position. + References No definition available. + Details
Name: | bmo_FairValueOfDerivativesInLiabilityPosition
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Loan commitment derivatives with notional. + 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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- Definition The cash inflow from the exercise of options. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 7 -IssueDate 2024-01-01 -Paragraph 17 -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=7&code=ifrs-tx-2024-en-r&anchor=para_17&doctype=Standard -URIDate 2024-03-27 + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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