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Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value $ 10,604 $ 10,670
Held-to-maturity Securities 10,519 10,644
Repurchase agreements 1,404 1,403
Long-term debt 12,300 12,610
Short-term debt 300 500
Trust preferred securities 308 308
Total liabilities 111,743 111,511
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 518 555
Repurchase agreements 0 0
Long-term Debt, Fair Value 0 0
Short-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 10,074 10,102
Repurchase agreements 1,404 1,403
Long-term Debt, Fair Value 12,757 12,998
Short-term Debt, Fair Value 312 503
Trust preferred securities 458 456
Total liabilities 14,931 15,360
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 12 13
Repurchase agreements 0 0
Long-term Debt, Fair Value 0 0
Short-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 518 555
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 90 106
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 651 667
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 2,728 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 12 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 1,354 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 5,251 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 10,604 10,670
Repurchase agreements 1,404 1,403
Long-term Debt, Fair Value 12,757 12,998
Short-term Debt, Fair Value 312 503
Trust preferred securities 458 456
Total liabilities 14,931 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 608 661
Estimate of Fair Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 651 667
Estimate of Fair Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,740 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 1,354 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 5,251 5,119
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 10,519 10,644
Repurchase agreements 1,404 1,403
Long-term debt 12,300 12,610
Short-term debt 300 500
Trust preferred securities 308 308
Total liabilities 14,312 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 601 655
Reported Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 625 640
Reported Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 2,710 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 1,319 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 $ 5,264 $ 5,185