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Significant Subsidiaries - Summary of Significant Operating Subsidiaries (Detail) $ in Millions | 12 Months Ended
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Oct. 31, 2025 CAD ($)
AIR MILES Loyalty Inc. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | AIR MILES Loyalty Inc.
Head or principal office | Toronto, Canada
Book value of shares owned by the bank | $ 19
Bank of Montreal (China) Co. Ltd. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | Bank of Montreal (China) Co. Ltd.
Head or principal office | Beijing, China
Book value of shares owned by the bank | $ 497
Bank of Montreal Europe Public Limited Company [Member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | Bank of Montreal Europe Public Limited Company
Head or principal office | Dublin, Ireland
Book value of shares owned by the bank | $ 1,391
Bank of Montreal Holding Inc. and subsidiaries [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | Bank of Montreal Holding Inc. and subsidiaries
Head or principal office | Toronto, Canada
Book value of shares owned by the bank | $ 35,455
Bank of Montreal Mortgage Corporation [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | Bank of Montreal Mortgage Corporation
Head or principal office | Calgary, Canada
BMO Mortgage Corp. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Mortgage Corp.
Head or principal office | Vancouver, Canada
BMO Investments Inc. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Investments Inc.
Head or principal office | Toronto, Canada
BMO InvestorLine Inc. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO InvestorLine Inc.
Head or principal office | Toronto, Canada
BMO Nesbitt Burns Inc. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Nesbitt Burns Inc.
Head or principal office | Toronto, Canada
BMO Private Investment Counsel Inc. [Member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Private Investment Counsel Inc.
Head or principal office | Toronto, Canada
BMO Private Equity (Canada) Inc [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Private Equity (Canada) Inc.
Head or principal office | Toronto, Canada
BMO Capital Markets Limited [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Capital Markets Limited
Head or principal office | London, England
Book value of shares owned by the bank | $ 434
BMO Capital Partners Inc [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Capital Partners Inc.
Head or principal office | Toronto, Canada
Book value of shares owned by the bank | $ 1,067
BMO Financial Corp. and subsidiaries [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Financial Corp. and subsidiaries
Head or principal office | Chicago, United States
Book value of shares owned by the bank | $ 60,146
BMO Bank National Association [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Bank National Association
Head or principal office | Chicago, United States
BMO Capital Markets Corp. [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Capital Markets Corp.
Head or principal office | New York, United States
BMO Japan Securities Ltd [Member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Japan Securities Ltd.
Head or principal office | Tokyo, Japan
Book value of shares owned by the bank | $ 6
BMO Life Insurance Company and subsidiary [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Life Insurance Company and subsidiary
Head or principal office | Toronto, Canada
Book value of shares owned by the bank | $ 1,430
BMO Life Assurance Company [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Life Assurance Company
Head or principal office | Toronto, Canada
BMO Trust Company [member] |
Disclosure of subsidiaries [line items] |
Significant subsidiaries | BMO Trust Company
Head or principal office | Toronto, Canada
Book value of shares owned by the bank | $ 630

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