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Equity - Additional Information (Detail)
$ / shares in Units, $ in Millions, $ in Millions
 6 Months Ended 12 Months Ended
Nov. 12, 2025
CAD ($)
Aug. 25, 2024
Apr. 30, 2024
shares
Oct. 31, 2025
CAD ($)
shares
Oct. 31, 2025
USD ($)
$ / shares
Oct. 31, 2025
USD ($)
shares
Sep. 02, 2025
shares
Aug. 25, 2025
CAD ($)
shares
Jul. 29, 2025
USD ($)
Jan. 17, 2025
shares
Nov. 25, 2024
CAD ($)
shares
Oct. 31, 2024
shares
Disclosure of classes of share capital [line items]                        
Description of conversion of preferred share       In such an event, each preferred share or other equity instrument is convertible into common shares pursuant to an automatic conversion formula and a conversion price based on the greater of: (i) a floor price of $5.00 and (ii) the current market price of our common shares based on the volume weighted-average trading price of our common shares on the TSX. In such an event, each preferred share or other equity instrument is convertible into common shares pursuant to an automatic conversion formula and a conversion price based on the greater of: (i) a floor price of $5.00 and (ii) the current market price of our common shares based on the volume weighted-average trading price of our common shares on the TSX.              
Series Fifty Three Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares         $ 1,000              
Series Fifty Four Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares         750              
Series Fifty Five Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares         $ 1,000              
Series Fifty One of Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares       $ 1,000                
Series 49 of Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares       $ 750                
Stock Option Plan [member]                        
Disclosure of classes of share capital [line items]                        
Common shares reserved for potential issuance | shares       5,699,134   5,699,134           6,554,492
4.300% LRCNs, Series 1 [member]                        
Disclosure of classes of share capital [line items]                        
Interest Rate 4.30%                      
4.800% AT1 Capital Notes (NVCC) [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value           $ 500            
Interest Rate   4.80%   6.709% 6.709%              
Dividend reinvestment and share purchase plan [member]                        
Disclosure of classes of share capital [line items]                        
Common shares reserved for potential issuance | shares       39,864,838   39,864,838           39,864,838
Common shares [member]                        
Disclosure of classes of share capital [line items]                        
Discount from treasury for shares issued under dividend reinvestment plan                       2.00%
Series 33 NVCC [Member]                        
Disclosure of classes of share capital [line items]                        
Redeem all of our outstanding               $ 200        
Redeemable preference shares, outstanding | shares               8,000,000        
Series 31 NVCC [Member]                        
Disclosure of classes of share capital [line items]                        
Redeem all of our outstanding                     $ 300  
Redeemable preference shares, outstanding | shares                     12,000,000  
Class B Series 48 Preferred Shares [member]                        
Disclosure of classes of share capital [line items]                        
Redeem all of our outstanding $ 1,250                      
Series 48 of Non Viability Contingent Capital [Member]                        
Disclosure of classes of share capital [line items]                        
BMO issued Non-cumulative, 5-year Rate Reset Class B Preferred Shares       $ 1,250                
Common shares [member] | September 2025 Normal Course Issuer Bid [Member]                        
Disclosure of classes of share capital [line items]                        
Share Repurchase Program Number Of Shares Repurchased | shares       5,800,000   5,800,000            
Common shares [member] | January 2025 Normal Course Issuer Bid [Member]                        
Disclosure of classes of share capital [line items]                        
Share Repurchase Program Number Of Shares Repurchased | shares       16,400,000   16,400,000            
Common shares [member] | Normal Course Issuer Bid [Member]                        
Disclosure of classes of share capital [line items]                        
Share Repurchase Program Authorized Amount       $ 3,461                
Share Repurchase Program Price Per Share | $ / shares         $ 152.97              
Share Repurchase Program Number Of Shares Repurchased | shares       22,200,000   22,200,000            
Number of shares repurchase under normal course issuer bid | shares             30,000,000     20,000,000    
Other Equity Instruments [member] | 7.300% Series 5 LRCNs [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value           $ 750            
Interest Rate       7.30% 7.30%              
Other Equity Instruments [member] | 7.700% Series 4 LRCNs [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value           1,000            
Interest Rate       7.70% 7.70%              
Other Equity Instruments [member] | 4.300% LRCNs, Series 1 [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value       $ 1,250                
Interest Rate       4.30% 4.30%              
Redeem all of our outstanding $ 1,250                      
Other Equity Instruments [member] | 5.625% LRCNs, Series 2 [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value       $ 750                
Interest Rate       5.625% 5.625%              
Other Equity Instruments [member] | 6.875% LRCNs, Series 6 [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value           $ 1,000     $ 1,000      
Interest Rate       6.875% 6.875%              
Other Equity Instruments [member] | 7.325% LRCNs, Series 3 [member]                        
Disclosure of classes of share capital [line items]                        
Note issued face value       $ 1,000                
Interest Rate       7.325% 7.325%              
Shareholder Dividend Reinvestment and Share Purchase Plan [member] | Common shares [member]                        
Disclosure of classes of share capital [line items]                        
Number of shares issued under dividend reinvestment plan at 2% discount from treasury | shares     7,790,724