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Commitments, Guarantees, Pledged Assets, Provisions and Contingent Liabilities - Summary of Maximum Amounts Payable Under Various Commitments (Parenthetical) (Detail) - CAD ($) $ in Millions	12 Months Ended
	Oct. 31, 2025	Oct. 31, 2024
Disclosure commitments guarantees pledged assets provisions and contingent liabilities [line items]
Fair value of derivative liabilities	$ 1	$ 8
Other commitments	$ 1,664	$ 4,511

X

- Definition

Line items represent commitments guarantees pledged assets provisions and contingent liabilities.

+ References

No definition available.

+ Details

Name:	bmo_DisclosureCommitmentsGuaranteesPledgedAssetsProvisionsAndContingentLiabilitiesLineItems
Namespace Prefix:	bmo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The fair value of financial liabilities classified as credit default swaps.

+ References

No definition available.

+ Details

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

X

- Definition

Other Commitment .

+ References

No definition available.

+ Details

Name:	bmo_Othercommitment
Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant