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Securities - Summary of Non-Interest Revenue (Detail) - CAD ($) $ in Millions | 12 Months Ended
--- | ---
Oct. 31, 2025 | Oct. 31, 2024
Interest Dividend and Fee Income [abstract] |  |  |
FVTPL securities |  | $ 201 | $ 87
FVOCI securities – realized gains | [1] | 89 | 114
Impairment on FVOCI and amortized cost securities |  | (3) | (1)
Securities gains, other than trading |  | $ 287 | $ 200
[1] | Gains are net of (losses) on hedge contracts.

X
- Definition Impairment loss recovery on financial assets at fair value through other comprehensive income and amortized cost securities. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Interest Dividend and Fee Income [abstract] + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Non Trading Securities At Fair Value Through Profit Or Loss Non Interest Revenue + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Securities at fair value through other comprehensive income realized gains. + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
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- Definition Securities gains, other than trading. + References No definition available. + Details
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Namespace Prefix: | bmo_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration