Securities - Amortized Cost and Fair Value of Asset Backed Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2026	Dec. 31, 2025
Securities available-for-sale:
Amortized Cost	$ 34,311	$ 31,651
Unrealized Gains	30	56
Unrealized Losses	(1,790)	(1,677)
Fair Value	32,551	30,030
Securities held-to-maturity:
Amortized Cost	19,979	18,293
Unrealized Gains	79	140
Unrealized Losses	(368)	(296)
Fair Value	19,690	18,137
U.S. Treasury
Securities available-for-sale:
Amortized Cost	15,602	13,895
Unrealized Gains	9	32
Unrealized Losses	(173)	(107)
Fair Value	15,438	13,820
Securities held-to-maturity:
Amortized Cost	4,962	2,846
Unrealized Gains	0	7
Unrealized Losses	(32)	(4)
Fair Value	4,930	2,849
U.S. Government sponsored enterprises
Securities available-for-sale:
Amortized Cost	4,528	4,603
Unrealized Gains	2	2
Unrealized Losses	(813)	(808)
Fair Value	3,717	3,797
Securities held-to-maturity:
Amortized Cost	1,039	1,062
Unrealized Gains	2	3
Unrealized Losses	(49)	(47)
Fair Value	992	1,018
U.S. Government agency issued or guaranteed
Securities available-for-sale:
Amortized Cost	9,635	8,750
Unrealized Gains	11	11
Unrealized Losses	(793)	(752)
Fair Value	8,853	8,009
Securities held-to-maturity:
Amortized Cost	13,976	14,383
Unrealized Gains	77	130
Unrealized Losses	(287)	(245)
Fair Value	13,766	14,268
Home equity mortgages
Securities available-for-sale:
Amortized Cost	8	8
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	8	8
Corporate loans
Securities available-for-sale:
Amortized Cost	154	179
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	154	179
Other
Securities available-for-sale:
Amortized Cost	85	85
Unrealized Gains	0	0
Unrealized Losses	(9)	(9)
Fair Value	76	76
Foreign debt securities
Securities available-for-sale:
Amortized Cost	4,299	4,131
Unrealized Gains	8	11
Unrealized Losses	(2)	(1)
Fair Value	4,305	4,141
Obligations of U.S. states and political subdivisions
Securities held-to-maturity:
Amortized Cost	2	2
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	$ 2	$ 2