Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Financial Instrument Assets (Liabilities) (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Financial assets:
Interest-bearing deposits at banks	$ 28,069	$ 24,959
Federal funds sold		3
Investment securities	26,897	25,211
Loans and leases, net	131,939	129,639
Financial liabilities:
Noninterest-bearing deposits	(49,294)	(65,502)
Savings and interest-checking deposits	(93,221)	(87,911)
Time deposits	(20,759)	(10,102)
Long-term borrowings	(8,201)	(3,965)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,668	1,372
Trading account	101	118
Investment securities	258	145
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	63	145
Interest-bearing deposits at banks	28,069	24,959
Federal funds sold		3
Trading account	5
Investment securities	25,571	23,860
Loans and leases, net	7,240	7,180
Accrued interest receivable	786	646
Financial liabilities:
Noninterest-bearing deposits	(49,294)	(65,502)
Savings and interest-checking deposits	(93,221)	(87,911)
Time deposits	(20,715)	(10,143)
Short-term borrowings	(5,316)	(3,555)
Long-term borrowings	(8,107)	(3,926)
Accrued interest payable	(482)	(81)
Other financial instruments:
Commitments to sell real estate loans	30	54
Interest rate swap agreements used for interest rate risk management	10	(8)
Interest rate and foreign exchange contracts not designated as hedging instruments	(642)	(920)
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities	46	51
Loans and leases, net	121,898	119,048
Other financial instruments:
Commitments to originate real estate loans for sale	(17)	(46)
Other credit-related commitments	154	(149)
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,731	1,517
Interest-bearing deposits at banks	28,069	24,959
Federal funds sold		3
Trading account	106	118
Investment securities	26,897	25,211
Loans and leases, net	131,939	129,639
Accrued interest receivable	786	646
Financial liabilities:
Noninterest-bearing deposits	(49,294)	(65,502)
Savings and interest-checking deposits	(93,221)	87,911
Time deposits	(20,759)	(10,102)
Short-term borrowings	(5,316)	(3,555)
Long-term borrowings	(8,201)	(3,965)
Accrued interest payable	(482)	(81)
Other financial instruments:
Commitments to originate real estate loans for sale	(17)	(46)
Commitments to sell real estate loans	30	54
Other credit-related commitments	154	149
Interest rate swap agreements used for interest rate risk management	10	(8)
Interest rate and foreign exchange contracts not designated as hedging instruments	(642)	(920)
Cumulative Amount of Fair Value Hedging Adjustment Increasing (Decreasing) the Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,731	1,517
Interest-bearing deposits at banks	28,069	24,959
Federal funds sold		3
Trading account	106	118
Investment securities	25,875	24,056
Loans and leases, net	129,138	126,228
Accrued interest receivable	786	646
Financial liabilities:
Noninterest-bearing deposits	(49,294)	(65,502)
Savings and interest-checking deposits	(93,221)	87,911
Time deposits	(20,715)	(10,143)
Short-term borrowings	(5,316)	(3,555)
Long-term borrowings	(8,107)	(3,926)
Accrued interest payable	(482)	(81)
Other financial instruments:
Commitments to originate real estate loans for sale	(17)	(46)
Commitments to sell real estate loans	30	54
Other credit-related commitments	(154)	149
Interest rate swap agreements used for interest rate risk management	10	(8)
Interest rate and foreign exchange contracts not designated as hedging instruments	$ (642)	$ (920)

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