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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, 2018
Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value $ 13,259 $ 14,474
Debt Securities, Held-to-maturity 13,435 14,335
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,418 1,408
Short-term debt 509 1,013
Long-term debt 12,087 11,556
Trust preferred securities 308 308
Total liabilities 117,459 115,850
US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,182 915
Debt Securities, Held-to-maturity 1,185 908
Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,542 1,757
Debt Securities, Held-to-maturity 1,549 1,738
Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,508 3,219
Debt Securities, Held-to-maturity 2,601 3,159
Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,486 2,742
Debt Securities, Held-to-maturity 2,524 2,724
States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,541 5,841
Debt Securities, Held-to-maturity 5,576 5,806
Estimate of Fair Value Measurement [Member]    
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,418 1,408
Short-term debt, Fair Value 516 1,013
Long-term debt, Fair Value 12,181 12,332
Trust preferred securities 409 468
Total liabilities 14,524 15,221
Reported Value Measurement    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 13,435 14,335
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,418 1,408
Short-term debt 509 1,013
Long-term debt 12,087 11,556
Trust preferred securities 308 308
Total liabilities 14,322 14,285
Reported Value Measurement | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,185 908
Reported Value Measurement | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,549 1,738
Reported Value Measurement | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 2,601 3,159
Reported Value Measurement | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 2,524 2,724
Reported Value Measurement | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 5,576 5,806
Level 1    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,128 857
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 1,128 857
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 12,100 13,582
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,418 1,408
Short-term debt, Fair Value 516 1,013
Long-term debt, Fair Value 12,181 12,332
Trust preferred securities 409 468
Total liabilities 14,524 15,221
Level 2 | US Treasury and Government [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 54 58
Level 2 | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,542 1,757
Level 2 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,477 3,184
Level 2 | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,486 2,742
Level 2 | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,541 5,841
Level 3    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 31 35
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 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 31 35
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