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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, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value $ 10,365 $ 10,670
Held-to-maturity Securities 10,160 10,644
Repurchase agreements 1,408 1,403
Short-term debt 1,020 500
Long-term debt 11,559 12,610
Trust preferred securities 308 308
Total liabilities 117,107 111,511
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 563 555
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
Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 9,791 10,102
Repurchase agreements 1,408 1,403
Short-term Debt, Fair Value 1,025 503
Long-term Debt, Fair Value 12,273 12,998
Trust preferred securities 467 456
Total liabilities 15,173 15,360
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 11 13
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
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 563 555
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 58 106
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Foreign [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Foreign [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 644 667
Foreign [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Corporate [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Corporate [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,578 2,782
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 11 13
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,258 1,428
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
States, Municipalities, And Political Subdivisions [Member] | Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
States, Municipalities, And Political Subdivisions [Member] | Fair Value, Inputs, Level 2 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 5,253 5,119
States, Municipalities, And Political Subdivisions [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 10,365 10,670
Repurchase agreements 1,408 1,403
Short-term Debt, Fair Value 1,025 503
Long-term Debt, Fair Value 12,273 12,998
Trust preferred securities 467 456
Total liabilities 15,173 15,360
Estimate of Fair Value Measurement [Member] | U.S. Treasury And Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 621 661
Estimate of Fair Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 644 667
Estimate of Fair Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,589 2,795
Estimate of Fair Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,258 1,428
Estimate of Fair Value Measurement [Member] | States, Municipalities, And Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 5,253 5,119
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 10,160 10,644
Repurchase agreements 1,408 1,403
Short-term debt 1,020 500
Long-term debt 11,559 12,610
Trust preferred securities 308 308
Total liabilities 14,295 14,821
Reported Value Measurement [Member] | U.S. Treasury And Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 611 655
Reported Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 616 640
Reported Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 2,531 2,771
Reported Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 1,223 1,393
Reported Value Measurement [Member] | States, Municipalities, And Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities $ 5,179 $ 5,185