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Insurance - Summary of Sensitivity Analysis to Changes in Risk Exposures that Arise from Contracts Within Scope of IFRS 17 (Detail) - CAD ($) $ in Millions | 12 Months Ended
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Oct. 31, 2025 | Oct. 31, 2024
Disclosure of sensitivity analysis to changes in risk variables that arise from contracts within scope of IFRS 17 [line items] |  |
50 basis point increase | $ 2 | $ 6
50 basis point decrease | (6) | (9)
10% Increase | 6 | 28
10% Decrease | $ (7) | $ (26)

X
- Definition Decrease in profit or loss from fifty basis point decrease in interest rates. + References No definition available. + Details
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Period Type: | duration

X
- Definition Decrease in profit or loss from 10 percent decrease in equity prices. + References No definition available. + Details
Name: | bmo_DecreaseInProfitOrLossFromTenPercentDecreaseInEquityPrices
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Increase in profit or loss from fifty basis point increase in interest rates. + References No definition available. + Details
Name: | bmo_IncreaseInProfitOrLossFromFiftyBasisPointIncreaseInInterestRates
Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Increase in profit or loss from 10 percent increase in equity prices. + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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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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