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Loans and Allowance for Credit Losses - Summary of Continuity in Loss Allowance by Each Product Type (Parenthetical) (Detail) - CAD ($) $ in Millions	12 Months Ended
	Oct. 31, 2025	Oct. 31, 2024
Other assets [member]
Disclosure of credit risk exposure [line items]
Provision for credit losses	$ (1)	$ 10

X

- Definition

Provision for credit losses.

+ References

No definition available.

+ Details

Name:	bmo_ProvisionForCreditLosses
Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

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

X

- Details

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