Fair Value Measurements - Carrying Amounts and Estimated Fair Value for Certain Financial Instruments (Detail) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Financial assets:
Interest-bearing deposits at banks	$ 32,144	$ 28,069
Investment securities held to maturity	15,078	15,330
Equity and other securities	928	859
Loans and leases, net	132,782	131,939
Financial liabilities:
Noninterest-bearing deposits	50,578	49,294
Savings and interest-checking deposits	96,339	93,221
Time deposits	20,279	20,759
Short-term borrowings	295	316
Long-term borrowings	11,450	8,201
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Financial assets:
Cash and cash equivalents	1,396	1,668
Significant Observable Inputs (Level 2) [Member]
Financial assets:
Cash and cash equivalents	299	63
Interest-bearing deposits at banks	32,144	28,069
Investment securities held to maturity	13,821	14,262
Loans and leases, net	7,354	7,240
Financial liabilities:
Time deposits	20,236	20,715
Short-term borrowings	4,795	5,316
Long-term borrowings	11,370	8,107
Significant Unobservable Inputs (Level 3) [Member]
Financial assets:
Investment securities held to maturity	44	46
Loans and leases, net	122,417	121,898
Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,695	1,731
Interest-bearing deposits at banks	32,144	28,069
Investment securities held to maturity	15,078	15,330
Loans and leases, net	132,782	131,939
Financial liabilities:
Time deposits	20,279	20,759
Short-term borrowings	4,795	5,316
Long-term borrowings	11,450	8,201
Cumulative Amount of Fair Value Hedging Adjustment Increasing (Decreasing) the Carrying Amount [Member]
Financial assets:
Cash and cash equivalents	1,695	1,731
Interest-bearing deposits at banks	32,144	28,069
Investment securities held to maturity	13,865	14,308
Loans and leases, net	129,771	129,138
Financial liabilities:
Time deposits	20,236	20,715
Short-term borrowings	4,795	5,316
Long-term borrowings	$ 11,370	$ 8,107