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Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($)
$ in Millions
Jun. 30, 2018
Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value $ 13,665 $ 14,474
Held-to-maturity Securities 13,860 14,335
Repurchase agreements 1,413 1,408
Short-term debt 600 1,013
Long-term debt 12,184 11,556
Trust preferred securities 308 308
Total liabilities 116,563 115,850
Fair Value, Inputs, Level 1 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,079 857
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 12,554 13,582
Repurchase agreements 1,413 1,408
Short-term Debt, Fair Value 615 1,013
Long-term Debt, Fair Value 12,296 12,332
Trust preferred securities 433 468
Total liabilities 14,757 15,221
Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 32 35
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 1,079 857
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 54 58
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 1,675 1,757
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 2,633 3,184
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 32 35
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 2,585 2,742
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 5,607 5,841
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 13,665 14,474
Repurchase agreements 1,413 1,408
Short-term Debt, Fair Value 615 1,013
Long-term Debt, Fair Value 12,296 12,332
Trust preferred securities 433 468
Total liabilities 14,757 15,221
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 1,133 915
Estimate of Fair Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,675 1,757
Estimate of Fair Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,665 3,219
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 2,585 2,742
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 5,607 5,841
Reported Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 13,860 14,335
Repurchase agreements 1,413 1,408
Short-term debt 600 1,013
Long-term debt 12,184 11,556
Trust preferred securities 308 308
Total liabilities 14,505 14,285
Reported Value Measurement [Member] | U.S. Treasury And Agency [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 1,143 908
Reported Value Measurement [Member] | Foreign [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 1,678 1,738
Reported Value Measurement [Member] | Corporate [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 2,736 3,159
Reported Value Measurement [Member] | Collateralized Mortgage Backed Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 2,640 2,724
Reported Value Measurement [Member] | States, Municipalities, And Political Subdivisions [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities $ 5,663 $ 5,806