Earnings Per Share - Summary of Basic and Diluted Earnings Per Share (Parenthetical) (Detail) - Dilutive potential shares from stock options [Member] - $ / shares	3 Months Ended
Jan. 31, 2025	Jan. 31, 2024
Earnings per share [line items]
Outstanding options excluded from calculation of diluted earnings per share	482,948	2,991,066
Weighted-average exercise price	$ 153.89	$ 132.29

Weighted average exercise price of average outstanding share options excluded from calculation of diluted earnings per share.

No definition available.

Name:	bmo_WeightedAverageExercisePriceOfAverageOutstandingShareOptionsExcludedFromCalculationOfDilutedEarningsPerShare
Namespace Prefix:	bmo_
Data Type:	dtr-types:perShareItemType
Balance Type:	na
Period Type:	duration