Income Taxes (Details) $ in Millions	6 Months Ended
Apr. 30, 2026 CAD ($)
Disclosure of Income Tax [Abstract]
Additional income tax expense and interest	$ 1,465

Additional income tax expense and interest from CRA reassessment.

No definition available.

Name:	bmo_AdditionalIncomeTaxAndInterestAssessed
Namespace Prefix:	bmo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration