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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, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value $ 14,474 $ 10,670
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 14,335 10,644
Repurchase agreements 1,408 1,403
Short-term debt 1,013 500
Long-term debt 11,556 12,610
Trust preferred securities 308 308
Total liabilities 115,850 111,511
US Treasury and Government [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 915 661
Foreign    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,757 667
Corporate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 3,219 2,795
Mortgage backed-securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,742 1,428
US States and Political Subdivisions Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 5,841 5,119
Estimate of Fair Value Measurement [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Repurchase agreements 1,408 1,403
Short-term debt, Fair Value 1,013 503
Long-term debt, Fair Value 12,332 12,998
Trust preferred securities 468 456
Total liabilities 15,221 15,360
Reported Value Measurement    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 14,335 10,644
Repurchase agreements 1,408 1,403
Short-term debt 1,013 500
Long-term debt 11,556 12,610
Trust preferred securities 308 308
Total liabilities 14,285 14,821
Reported Value Measurement | US Treasury and Government [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 908 655
Reported Value Measurement | Foreign    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 1,738 640
Reported Value Measurement | Corporate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 3,159 2,771
Reported Value Measurement | Mortgage backed-securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 2,724 1,393
Reported Value Measurement | US States and Political Subdivisions Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Fixed maturities held to maturity, at amortized cost (fair value – $14,474 and $10,670) 5,806 5,185
Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 857 555
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
Level 1 | US Treasury and Government [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 857 555
Level 1 | Foreign    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 1 | Corporate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 1 | Mortgage backed-securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 1 | US States and Political Subdivisions Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 13,582 10,102
Repurchase agreements 1,408 1,403
Short-term debt, Fair Value 1,013 503
Long-term debt, Fair Value 12,332 12,998
Trust preferred securities 468 456
Total liabilities 15,221 15,360
Level 2 | US Treasury and Government [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 58 106
Level 2 | Foreign    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 1,757 667
Level 2 | Corporate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 3,184 2,782
Level 2 | Mortgage backed-securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 2,742 1,428
Level 2 | US States and Political Subdivisions Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 5,841 5,119
Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 35 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
Level 3 | US Treasury and Government [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 3 | Corporate securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 35 13
Level 3 | Mortgage backed-securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value 0 0
Level 3 | US States and Political Subdivisions Debt Securities [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities, Fair Value $ 0 $ 0