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Deposits - Additional Information (Detail) - CAD ($) $ in Millions | Oct. 31, 2025 | Oct. 31, 2024
--- | --- | ---
Disclosure Of Deposits [Line Items] |  |
Deposits eligible for early redemption without penalty | $ 27,819 | $ 29,136
Commercial paper | 43,617 | 51,500
Covered bonds | $ 24,053 | $ 26,957

X
- Definition Commercial papers issued by the entity. + References No definition available. + Details
Name: | bmo_CommercialPaper1
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Deposit eligible for early redemption without penalty. + References No definition available. + Details
Name: | bmo_DepositEligibleForEarlyRedemptionWithoutPenalty
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Disclosure Of Deposits [Line Items] + References No definition available. + Details
Name: | bmo_DisclosureOfDepositsLineItems
Namespace Prefix: | bmo_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of bonds issued by the entity. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2024-01-01 -Paragraph 112 -Subparagraph c -URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2024-en-r&anchor=para_112_c&doctype=Standard -URIDate 2024-03-27 + Details
Name: | ifrs-full_BondsIssued
Namespace Prefix: | ifrs-full_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant