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IFRS 7 Disclosure - Credit and Counterparty Risk - Additional Information (Detail) - CAD ($) $ in Millions	12 Months Ended
	Oct. 31, 2025	Oct. 31, 2024
Disclosure of credit risk exposure [line items]
Credit assets exposure value	$ 360,814	$ 353,309
Outstanding credit exposures	1,189,600
Canada [member]
Disclosure of credit risk exposure [line items]
Outstanding credit exposures	596,400
United States [member]
Disclosure of credit risk exposure [line items]
Outstanding credit exposures	542,700
Other Jurisdications [member]
Disclosure of credit risk exposure [line items]
Outstanding credit exposures	$ 50,500
Credit risk [member] | Residential real estate loan [member]
Disclosure of credit risk exposure [line items]
Loan to value ratio	80.00%

X

- Definition

Loan to value ratio.

+ References

No definition available.

+ Details

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Namespace Prefix:	bmo_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Outstanding credit exposures.

+ References

No definition available.

+ Details

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Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- 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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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The maximum exposure to the credit risk of financial assets designated as measured at fair value through profit or loss. [Refer: Credit risk [member]; Maximum exposure to credit risk]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 9
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_9_a&doctype=Standard
-URIDate 2024-03-27

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Namespace Prefix:	ifrs-full_
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Period Type:	instant

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- Details

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Namespace Prefix:
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Balance Type:
Period Type:

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- Details

Name:	ifrs-full_GeographicalAreasAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	ifrs-full_GeographicalAreasAxis=bmo_OtherJurisdicationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: