Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 12,397	$ 10,691
Investment securities available for sale, gross unrealized gains	1
Investment securities available for sale, gross unrealized losses	264	251
Investment securities available for sale, estimated fair value	12,134	10,440
Amortized cost for held to maturity	15,078	15,330
Gross unrealized gains for held to maturity	9	13
Gross unrealized losses for held to maturity	1,222	1,035
Estimated fair value for held to maturity	13,865	14,308
Equity and other securities, Amortized Cost	1,279	1,125
Equity securities, Gross Unrealized Gains	8	5
Equity securities, Gross Unrealized Losses	3	3
Equity and other securities, Estimated Fair Value	1,284	1,127
Other securities, Amortized cost	928	859
Other securities, Estimated fair value	928	859
Total debt securities Amortized cost	27,475	26,021
Total debt securities Gross unrealized gains	10	13
Total debt securities Gross unrealized losses	1,486	1,286
Total debt securities Estimated fair value	25,999	24,748
Readily marketable securities Amortized cost	351	266
Readily marketable securities Gross unrealized gains	8	5
Readily marketable securities Gross unrealized losses	3	3
Readily marketable securities Estimated fair value	356	268
U.S. Treasury and Federal Agencies [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	7,818	7,818
Investment securities available for sale, gross unrealized losses	99	113
Investment securities available for sale, estimated fair value	7,719	7,705
Amortized cost for held to maturity	1,007	1,005
Gross unrealized losses for held to maturity	33	31
Estimated fair value for held to maturity	974	974
Obligations of States and Political Subdivisions [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	2,466	2,501
Gross unrealized losses for held to maturity	94	67
Estimated fair value for held to maturity	2,372	2,434
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	1,355	425
Investment securities available for sale, gross unrealized gains	1
Investment securities available for sale, gross unrealized losses	13	9
Investment securities available for sale, estimated fair value	1,343	416
Amortized cost for held to maturity	2,035	2,033
Gross unrealized losses for held to maturity	154	130
Estimated fair value for held to maturity	1,881	1,903
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	3,053	2,272
Investment securities available for sale, gross unrealized losses	143	118
Investment securities available for sale, estimated fair value	2,910	2,154
Amortized cost for held to maturity	9,527	9,747
Gross unrealized gains for held to maturity		4
Gross unrealized losses for held to maturity	935	802
Estimated fair value for held to maturity	8,592	8,949
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	171	176
Investment securities available for sale, gross unrealized losses	9	11
Investment securities available for sale, estimated fair value	162	165
Amortized cost for held to maturity	2	2
Estimated fair value for held to maturity	2	2
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Amortized cost for held to maturity	41	42
Gross unrealized gains for held to maturity	9	9
Gross unrealized losses for held to maturity	6	5
Estimated fair value for held to maturity	$ 44	$ 46

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