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Contractual Maturities of Assets and Liabilities and Off-Balance Sheet Commitments - Summary of Remaining Contractual Maturity (Parenthetical) (Detail) - CAD ($) $ in Millions | Oct. 31, 2025 | Oct. 31, 2024
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Disclosure of assets and liabilities according to their remaining contractual maturities [Abstract] |  |
Deposits eligible for early redemption without penalty | $ 27,819 | $ 29,136

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- Definition Deposits eligible for early redemption without penalty. + References No definition available. + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Disclosure of assets and liabilities according to their remaining contractual maturities [Abstract] + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration