Transfers of Financial Assets - Additional Information (Detail) - CAD ($) $ in Millions	12 Months Ended
Oct. 31, 2025	Oct. 31, 2024
Disclosure of transferred financial assets that are not derecognised in their entirety [line items]
Sale of mortgage loans	$ 4,621	$ 3,687
Securities lent	15,011	12,913
Collateralized Mortgage Obligations at fair value	6	6
Securities Sold Under Repurchase Agreement	119,956	97,878
United States [member]
Disclosure of transferred financial assets that are not derecognised in their entirety [line items]
Sale of mortgage loans	849	10,249
Carry value of Mortgage servicing rights	146	169
Fair value of Mortgage serving rights	170	192
Gain of derecognised financial assets at date of transfer	24	(153)
United States [member] \| Residual Interests of Commercial Mortgage Loans [Member]
Disclosure of transferred financial assets that are not derecognised in their entirety [line items]
Proceeds from continuing involvement in derecognised financial assets	3,863	4,412
Gain of derecognised financial assets at date of transfer	44	49
Debt securities at amortized cost	7	7
Loans carried at amortized cost	92	40
NHA MBS Program [Member] \| Canada [member] \| Residential Mortgages [Member]
Disclosure of transferred financial assets that are not derecognised in their entirety [line items]
Sale of mortgage loans	3,019	2,157
Collateralized Mortgage Obligations at fair value	145	146
Gain of derecognised financial assets at date of transfer	$ (1)	$ 90

Collateralized Mortgage Obligations at fair value.

No definition available.

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- Definition

Fair value of mortgage serving rights.

+ References

No definition available.

+ Details

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- Definition

Mortgage servicing rights.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Proceeds from sale of mortgage loan securitization.

+ References

No definition available.

+ Details

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Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
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- Definition

Securities lent.

+ References

No definition available.

+ Details

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- Definition

Securities Sold Under Repurchase Agreement.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

Name:	ifrs-full_DisclosureOfFinancialAssetsTransferredDuringPeriodWhichDoNotQualifyForDerecognitionLineItems
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- Definition

The fair value of assets representing the entity's continuing involvement in derecognised financial assets. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 42E
-Subparagraph b
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_42E_b&doctype=Standard
-URIDate 2024-03-27

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Balance Type:	debit
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- Definition

The gain (loss) recognised on derecognised financial assets as of the date of the transfer of the assets. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 42G
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_42G_a&doctype=Standard
-URIDate 2024-03-27

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- Definition

The amount of proceeds recognised from the transfer of financial assets during the part of the reporting period within which the greatest transfer activity took place when the total amount of proceeds from the transfer activity (that qualifies for derecognition) is not evenly distributed throughout the reporting period. [Refer: Financial assets]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 42G
-Subparagraph c
-Clause iii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_42G_c_iii&doctype=Standard
-URIDate 2024-03-27

+ Details

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- Definition

The amount of assets recognised in the statement of financial position representing the entity's continuing involvement in derecognised financial assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IFRS
-Number 7
-IssueDate 2024-01-01
-Paragraph 42E
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-URI https://taxonomy.ifrs.org/xifrs-link?type=IFRS&num=7&code=ifrs-tx-2024-en-r&anchor=para_42E_a&doctype=Standard
-URIDate 2024-03-27

+ Details

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- Details

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- Details

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- Details

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- Details

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