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Significant Subsidiaries - Summary of Significant Operating Subsidiaries (Detail) $ in Millions	12 Months Ended
	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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