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Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value $ 13,284 $ 14,474
Debt Securities, Held-to-maturity 13,563 14,335
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,414 1,408
Short-term debt 500 1,013
Long-term debt 12,149 11,556
Trust preferred securities 308 308
Total liabilities 116,750 115,850
U.S. Treasury and agency    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,093 915
Debt Securities, Held-to-maturity 1,112 908
Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,586 1,757
Debt Securities, Held-to-maturity 1,601 1,738
Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,578 3,219
Debt Securities, Held-to-maturity 2,658 3,159
Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,503 2,742
Debt Securities, Held-to-maturity 2,574 2,724
States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,524 5,841
Debt Securities, Held-to-maturity 5,618 5,806
Level 1    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,039 857
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 | U.S. Treasury and agency    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,039 857
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 12,214 13,582
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,414 1,408
Short-term Debt, Fair Value 510 1,013
Long-term Debt, Fair Value 12,224 12,332
Trust preferred securities 432 468
Total liabilities 14,580 15,221
Level 2 | U.S. Treasury and agency    
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 1,586 1,757
Level 2 | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,547 3,184
Level 2 | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,503 2,742
Level 2 | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,524 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 | U.S. Treasury and agency    
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 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
Estimate of Fair Value Measurement [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 13,284 14,474
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,414 1,408
Short-term Debt, Fair Value 510 1,013
Long-term Debt, Fair Value 12,224 12,332
Trust preferred securities 432 468
Total liabilities 14,580 15,221
Estimate of Fair Value Measurement [Member] | U.S. Treasury and agency    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,093 915
Estimate of Fair Value Measurement [Member] | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 1,586 1,757
Estimate of Fair Value Measurement [Member] | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,578 3,219
Estimate of Fair Value Measurement [Member] | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 2,503 2,742
Estimate of Fair Value Measurement [Member] | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity, Fair Value 5,524 5,841
Reported Value Measurement [Member]    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 13,563 14,335
Financial Instruments, Financial Liabilities, Balance Sheet Groupings [Abstract]    
Repurchase agreements 1,414 1,408
Short-term debt 500 1,013
Long-term debt 12,149 11,556
Trust preferred securities 308 308
Total liabilities 14,371 14,285
Reported Value Measurement [Member] | U.S. Treasury and agency    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,112 908
Reported Value Measurement [Member] | Foreign    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 1,601 1,738
Reported Value Measurement [Member] | Corporate securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 2,658 3,159
Reported Value Measurement [Member] | Mortgage-backed securities    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity 2,574 2,724
Reported Value Measurement [Member] | States, municipalities, and political subdivisions    
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]    
Debt Securities, Held-to-maturity $ 5,618 $ 5,806