Investment securities - Investment Securities in Continuous Unrealized Loss Position (Detail) - USD ($) $ in Millions	Sep. 30, 2025	Dec. 31, 2024
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	$ 594	$ 8,966
AFS, less than 12 months, gross unrealized losses	2	107
AFS, 12 months or longer, fair value	2,012	4,243
AFS, 12 months or longer, gross unrealized losses	71	132
Total investment securities, less than 12 months, fair value	688	10,051
Total investment securities, less than 12 months, gross unrealized losses	3	127
Total investment securities, 12 months or longer, fair value	12,822	16,013
Total investment securities, 12 months or longer, gross unrealized losses	876	1,361
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	94	1,085
HTM, Less than 12 months, gross unrealized losses	1	20
HTM, 12 months or longer, fair value	10,810	11,770
HTM, 12 months or longer, gross unrealized losses	805	1,229
U.S. Treasury
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	0	1,971
AFS, less than 12 months, gross unrealized losses	0	9
AFS, 12 months or longer, fair value	247	2,554
AFS, 12 months or longer, gross unrealized losses	3	18
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	0	50
HTM, Less than 12 months, gross unrealized losses	0	0
HTM, 12 months or longer, fair value	438	951
HTM, 12 months or longer, gross unrealized losses	6	14
Commercial
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	135	2,566
AFS, less than 12 months, gross unrealized losses	0	45
AFS, 12 months or longer, fair value	323	64
AFS, 12 months or longer, gross unrealized losses	2	0
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	0	0
HTM, Less than 12 months, gross unrealized losses	0	0
HTM, 12 months or longer, fair value	1,943	1,877
HTM, 12 months or longer, gross unrealized losses	77	157
Residential
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	458	4,429
AFS, less than 12 months, gross unrealized losses	2	53
AFS, 12 months or longer, fair value	1,442	1,623
AFS, 12 months or longer, gross unrealized losses	66	114
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	83	996
HTM, Less than 12 months, gross unrealized losses	1	19
HTM, 12 months or longer, fair value	6,513	6,811
HTM, 12 months or longer, gross unrealized losses	661	942
Other
Debt Securities, Available-for-Sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, less than 12 months, fair value	1	0
AFS, less than 12 months, gross unrealized losses	0	0
AFS, 12 months or longer, fair value	0	2
AFS, 12 months or longer, gross unrealized losses	0	0
State and political subdivisions
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	9	39
HTM, Less than 12 months, gross unrealized losses	0	1
HTM, 12 months or longer, fair value	1,916	2,131
HTM, 12 months or longer, gross unrealized losses	61	$ 116
Privately issued
Schedule of Held-to-Maturity Securities [Line Items]
HTM, less than 12 months, fair value	2
HTM, Less than 12 months, gross unrealized losses	0
HTM, 12 months or longer, fair value	0
HTM, 12 months or longer, gross unrealized losses	$ 0