v3.26.1
Loans and Related Allowance for Credit Losses (Rollforward of Allowance for Credit Losses of Loans and Leases) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Dec. 31, 2025
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance $ 4,410 $ 4,486 $ 4,486
Acquisition PCD reserves 93    
Acquisition PSL reserves 229    
Charge-offs (309) (312)  
Recoveries 101 107  
Acquired gross charge-offs (45) 0  
Net (charge-offs) (253) (205)  
Provision for credit losses 188 260  
Other (4) 3  
Ending balance 4,663 4,544 4,410
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]      
Beginning balance 818 719 719
Provision for (recapture of) credit losses 14 (46)  
Other 0 1  
Ending balance 832 674 818
Allowance for credit loss 5,495 5,218 5,200
Allowances for investment securities and other financial assets 103 91  
Cumulative Effect, Period of Adoption, Adjusted Balance      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance 4,732 4,486 4,486
Ending balance     4,732
FirstBank Holding Company      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Acquired gross charge-offs (45)    
Total commercial lending      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance 3,089 3,148 3,148
Acquisition PCD reserves 53    
Acquisition PSL reserves 184    
Charge-offs (148) (131) (510)
Recoveries 38 47  
Acquired gross charge-offs (10) 0  
Net (charge-offs) (120) (84)  
Provision for credit losses 67 138  
Other (4) 3  
Ending balance 3,269 3,205 3,089
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]      
Beginning balance 681 580 580
Provision for (recapture of) credit losses 12 (53)  
Other 0 1  
Ending balance 693 528 681
Allowance for credit loss 3,962 3,733  
Total commercial lending | Cumulative Effect, Period of Adoption, Adjusted Balance      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance 3,326 3,148 3,148
Ending balance     3,326
Total consumer lending      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance 1,321 1,338 1,338
Acquisition PCD reserves 40    
Acquisition PSL reserves 45    
Charge-offs (161) (181)  
Recoveries 63 60  
Acquired gross charge-offs (35) 0  
Net (charge-offs) (133) (121)  
Provision for credit losses 121 122  
Other 0 0  
Ending balance 1,394 1,339 1,321
Off-Balance Sheet, Credit Loss, Liability [Roll Forward]      
Beginning balance 137 139 139
Provision for (recapture of) credit losses 2 7  
Other 0 0  
Ending balance 139 146 137
Allowance for credit loss 1,533 1,485  
Total consumer lending | Cumulative Effect, Period of Adoption, Adjusted Balance      
Financing Receivable, Allowance for Credit Loss [Roll Forward]      
Beginning balance $ 1,406 $ 1,338 1,338
Ending balance     $ 1,406