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Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value $ 12,510 $ 13,005
Debt Securities, Held-to-maturity 11,697 12,581
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,405 1,416
Short-term debt 0 1,299
Long-term debt 14,948 13,559
Trust preferred securities 308 308
Total liabilities 131,333 121,612
US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,452 1,347
Debt Securities, Held-to-maturity 1,392 1,318
Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,405 1,485
Debt Securities, Held-to-maturity 1,295 1,423
Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,438 2,468
Debt Securities, Held-to-maturity 2,185 2,349
Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,146 2,396
Debt Securities, Held-to-maturity 2,000 2,331
States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,069 5,309
Debt Securities, Held-to-maturity 4,825 5,160
Estimate of Fair Value Measurement [Member]    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,405 1,416
Short-term debt, Fair Value 0 1,307
Long-term debt, Fair Value 17,487 15,048
Trust preferred securities 473 467
Total liabilities 19,365 18,238
Reported Value Measurement    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 11,653 12,581
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,405 1,416
Short-term debt 0 1,299
Long-term debt 14,948 13,559
Trust preferred securities 308 308
Total liabilities 16,661 16,582
Reported Value Measurement | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,392 1,318
Reported Value Measurement | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,288 1,423
Reported Value Measurement | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 2,150 2,349
Reported Value Measurement | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,999 2,331
Reported Value Measurement | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 4,824 5,160
Level 1    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,395 1,292
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 0 0
Short-term debt, Fair Value 0 0
Long-term debt, Fair Value 0 0
Trust preferred securities 0 0
Total liabilities 0 0
Level 1 | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,395 1,292
Level 1 | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 1 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 1 | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 1 | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 2    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 11,115 11,681
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,405 1,416
Short-term debt, Fair Value 0 1,307
Long-term debt, Fair Value 17,487 15,048
Trust preferred securities 473 467
Total liabilities 19,365 18,238
Level 2 | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 57 55
Level 2 | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,405 1,485
Level 2 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,438 2,436
Level 2 | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,146 2,396
Level 2 | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,069 5,309
Level 3    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 32
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 0 0
Short-term debt, Fair Value 0 0
Long-term debt, Fair Value 0 0
Trust preferred securities 0 0
Total liabilities 0 0
Level 3 | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 3 | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 3 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 32
Level 3 | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 0 0
Level 3 | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value $ 0 $ 0