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Employee Compensation - Pension and Other Employee Future Benefits - Summary of Cash payments in connection with employee future benefit plans (Detail) - CAD ($) $ in Millions | 12 Months Ended
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Oct. 31, 2025 | Oct. 31, 2024
Pension Plans [member] |  |
Disclosure of defined benefit plans and defined contribution plans [Line Items] |  |
Net contributions (refund) | $ 4 | $ (25)
Contributions to defined contribution plans | 306 | 290
Benefits paid directly to pensioners | 51 | 50
Cash payments made for employee future benefit plans | 361 | 315
Other employee future benefit plans [member] |  |
Disclosure of defined benefit plans and defined contribution plans [Line Items] |  |
Net contributions (refund) | 0 |
Contributions to defined contribution plans | 0 |
Benefits paid directly to pensioners | 47 | 45
Cash payments made for employee future benefit plans | $ 47 | $ 45

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- Definition The amount represents cash payments made by the company in connection with employee future benefit plans. + References No definition available. + Details
Name: | bmo_ContributionsToPlanByEmployer
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Contributions to plan by employer, defined contribution plans + References No definition available. + Details
Name: | bmo_ContributionsToPlanByEmployerDefinedContributionPlans
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Disclosure of defined benefit plans and defined contribution plans. + References No definition available. + Details
Name: | bmo_DisclosureOfDefinedBenefitPlansAndDefinedContributionPlansLineItems
Namespace Prefix: | bmo_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Net contributions (refund) to defined benefit plans. + References No definition available. + Details
Name: | bmo_NetContributionsRefundToPlanByEmployerDefinedBenefitPlans
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition The amount represents benefits paid by the company directly to pensioners. + References No definition available. + Details
Name: | bmo_PaymentsMadeByTheCompanyDirectlyToPensioners
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | bmo_DefinedBenefitAndContributionPlanAxis=bmo_PensionPlansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | bmo_DefinedBenefitAndContributionPlanAxis=bmo_OtherEmployeeFutureBenefitPlansMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |