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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
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

X

- 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

X

- 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

X

- Details

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

X

- Details

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

X

- Details

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