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Insurance - Summary of the Insurance Liability by Remaining Coverage and Incurred Claims (Parenthetical) (Detail) - CAD ($) $ in Millions	12 Months Ended
	Oct. 31, 2025	Oct. 31, 2024
Insurance service result [Line Items]
Liabilities for incurred claims	$ 102	$ 115
The Union Life [Member]
Insurance service result [Line Items]
Insurance contracts transferred	$ 798

X

- Definition

Insurance contracts transferred.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

Name:	bmo_InsuranceServiceResultLineItems
Namespace Prefix:	bmo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The amount of the liabilities for incurred claims that arise from contracts within the scope of IFRS 17. [Refer: Liabilities for incurred claims [member]]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 17
-IssueDate 2024-01-01
-Paragraph 130
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=17&code=ifrs-tx-2024-en-r&anchor=para_130&doctype=Standard
-URIDate 2024-03-27

+ Details

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Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	ifrs-full_InsuranceContractsAxis=bmo_TheUnionLifeMember
Namespace Prefix:
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Balance Type:
Period Type: