Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 10,691	$ 11,193
Investment securities available for sale, gross unrealized gains		1
Investment securities available for sale, gross unrealized losses	251	445
Investment securities available for sale, estimated fair value	10,440	10,749
Amortized cost for held to maturity	15,330	13,530
Gross unrealized gains for held to maturity	13	10
Gross unrealized losses for held to maturity	1,035	1,165
Estimated fair value for held to maturity	14,308	12,375
Equity and other securities, Amortized Cost	1,125	934
Equity securities, Gross Unrealized Gains	5	2
Equity securities, Gross Unrealized Losses	3	4
Equity and other securities, Estimated Fair Value	1,127	932
Other securities, Amortized cost	859	781
Other securities, Estimated fair value	859	781
Total debt securities Amortized cost	26,021	24,723
Total debt securities Gross unrealized gains	13	11
Total debt securities Gross unrealized losses	1,286	1,610
Total debt securities Estimated fair value	24,748	23,124
Readily marketable securities Amortized cost	266	153
Readily marketable securities Gross unrealized gains	5	2
Readily marketable securities Gross unrealized losses	3	4
Readily marketable securities Estimated fair value	268	151
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	7,914
Investment securities available for sale, gross unrealized losses	113	243
Investment securities available for sale, estimated fair value	7,705	7,671
Amortized cost for held to maturity	1,005	1,054
Gross unrealized losses for held to maturity	31	46
Estimated fair value for held to maturity	974	1,008
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	2,501	2,577
Gross unrealized losses for held to maturity	67	117
Estimated fair value for held to maturity	2,434	2,460
Commercial [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	425	595
Investment securities available for sale, gross unrealized losses	9	21
Investment securities available for sale, estimated fair value	416	574
Amortized cost for held to maturity	2,033	912
Gross unrealized losses for held to maturity	130	103
Estimated fair value for held to maturity	1,903	809
Residential [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	2,272	2,501
Investment securities available for sale, gross unrealized losses	118	171
Investment securities available for sale, estimated fair value	2,154	2,330
Amortized cost for held to maturity	9,747	8,935
Gross unrealized gains for held to maturity	4	1
Gross unrealized losses for held to maturity	802	891
Estimated fair value for held to maturity	8,949	8,045
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	176	183
Investment securities available for sale, gross unrealized gains		1
Investment securities available for sale, gross unrealized losses	11	10
Investment securities available for sale, estimated fair value	165	174
Amortized cost for held to maturity	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	42	50
Gross unrealized gains for held to maturity	9	9
Gross unrealized losses for held to maturity	5	8
Estimated fair value for held to maturity	$ 46	$ 51

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