Loans and allowance for loan losses - Loan and Leases Considered Nonaccrual and Interest Income Recognized on Loans (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Jun. 30, 2025	Jan. 01, 2025	Dec. 31, 2024	Jun. 30, 2024	Jan. 01, 2024
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	$ 1,056	$ 1,235	$ 1,056	$ 1,235
Amortized Cost without Allowance	456	691	456	691
Total	1,512	1,926	1,512	1,926	$ 1,573	$ 1,690	$ 1,690	$ 2,024	$ 2,166
Amortized Cost	1,512	1,926	1,512	1,926	1,573	1,690	1,690	2,024	2,166
Interest Income Recognized	27	17	67	64
Commercial and industrial
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	652	610	652	610
Amortized Cost without Allowance	108	200	108	200
Total	760	810	760	810	787	696	696	805	670
Amortized Cost	760	810	760	810	787	696	696	805	670
Interest Income Recognized	8	5	20	14
Commercial Real Estate | Commercial
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	217	348	217	348
Amortized Cost without Allowance	144	230	144	230
Total	361	578	361	578	376	468	468	707	869
Amortized Cost	361	578	361	578	376	468	468	707	869
Interest Income Recognized	11	5	28	31
Commercial Real Estate | Residential builder and developer
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	0	2	0	2
Amortized Cost without Allowance	0	0	0	0
Total	0	2	0	2	1	2	2	2	3
Amortized Cost	0	2	0	2	1	2	2	2	3
Interest Income Recognized	0	1	0	1
Commercial Real Estate | Other commercial construction
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	13	21	13	21
Amortized Cost without Allowance	8	63	8	63
Total	21	84	21	84	23	66	66	77	171
Amortized Cost	21	84	21	84	23	66	66	77	171
Interest Income Recognized	2	0	2	3
Residential Real Estate | Residential
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	105	135	105	135
Amortized Cost without Allowance	145	141	145	141
Total	250	276	250	276	265	279	279	260	270
Amortized Cost	250	276	250	276	265	279	279	260	270
Interest Income Recognized	4	4	11	11
Consumer | Home equity lines and loans
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	37	39	37	39
Amortized Cost without Allowance	40	43	40	43
Total	77	82	77	82	75	81	81	79	81
Amortized Cost	77	82	77	82	75	81	81	79	81
Interest Income Recognized	2	2	6	4
Consumer | Recreational finance
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	20	17	20	17
Amortized Cost without Allowance	9	11	9	11
Total	29	28	29	28	25	31	31	25	36
Amortized Cost	29	28	29	28	25	31	31	25	36
Interest Income Recognized	0	0	0	0
Consumer | Automobile
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	8	8	8	8
Amortized Cost without Allowance	2	3	2	3
Total	10	11	10	11	9	12	12	11	14
Amortized Cost	10	11	10	11	9	12	12	11	14
Interest Income Recognized	0	0	0	0
Consumer | Other
Financing Receivable, Impaired [Line Items]
Amortized Cost with Allowance	4	55	4	55
Amortized Cost without Allowance	0	0	0	0
Total	4	55	4	55	12	55	55	58	52
Amortized Cost	4	55	4	55	$ 12	$ 55	$ 55	$ 58	$ 52
Interest Income Recognized	$ 0	$ 0	$ 0	$ 0