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Loans and Allowance for Credit Losses - Summary of Loans that are Past Due but not Classified as Impaired (Parenthetical) (Detail) - Financial assets past due but not impaired [member] - CAD ($) $ in Millions | Oct. 31, 2025 | Oct. 31, 2024
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Disclosure of detailed information about financial instruments [line items] |  |
Total loans | $ 2,317 | $ 2,323
Maturity Between 90 to 180 Days Past Due [member] |  |
Disclosure of detailed information about financial instruments [line items] |  |
Total loans | $ 7 | $ 16

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- Definition Loans Past Due But Not Impaired + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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
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Balance Type: | na
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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |