Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Document Period End Date	Mar. 31, 2016
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	$ 11,280	$ 8,430
Unearned Premium Reserves Amortization, Next Three Months	11,580	8,552
Investments in partially-owned insurance companies	654	653
Repurchase Agreements	1,403	1,404
Short-term debt	500	0
Long-term debt	12,636	9,389
Trust preferred securities	308	307
Trust Preferred Securities Not Carried At Fair Value		307
Total liabilities	110,747	73,171
Total Liabilities Not Carried At Fair Value Not Carried At Fair Value		11,100
Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unearned Premium Reserves Amortization, Next Three Months	598	583
Short-term debt	0
Long-term debt	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]
Unearned Premium Reserves Amortization, Next Three Months	10,969	7,955
Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unearned Premium Reserves Amortization, Next Three Months	13	14
Short-term debt	0
Long-term debt	0	0
Trust preferred securities	0	0
Total liabilities	0	0
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unearned Premium Reserves Amortization, Next Three Months	11,280	8,430
Repurchase Agreements	1,403
Short-term debt	500
Long-term debt	12,636
Trust Preferred Securities Not Carried At Fair Value	308
Total Liabilities Not Carried At Fair Value Not Carried At Fair Value	14,847
U.S. Treasury And Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	723	745
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	598	583
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	125	162
U.S. Treasury And Agency [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
U.S. Treasury And Agency [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	700	733
Foreign [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	783	785
Foreign [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
Foreign [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	783	785
Foreign [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
Foreign [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	749	763
Corporate [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	3,033	3,056
Corporate [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
Corporate [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	3,020	3,042
Corporate [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	13	14
Corporate [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	2,951	3,054
Collateralized Mortgage Backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	1,716	1,743
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	1,716	1,743
Collateralized Mortgage Backed Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
Collateralized Mortgage Backed Securities [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	1,652	1,707
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 – $11,580 and $8,552)	5,325	2,223
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	5,325	2,223
US States and Political Subdivisions Debt Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	0	0
US States and Political Subdivisions Debt Securities [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $11,580 and $8,552)	5,228	2,173
Financial Instruments Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase Agreements	1,403	1,404
Short-term Debt, Fair Value	519
Long-term debt		9,678
Long-term Debt, Fair Value	13,317
Trust Preferred Securities Not Carried At Fair Value	433	446
Total Liabilities Not Carried At Fair Value Not Carried At Fair Value	15,672	11,528
Financial Instruments Fair Value [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Repurchase Agreements	1,403	1,404
Short-term Debt, Fair Value	519
Long-term Debt, Fair Value	13,317	9,678
Trust Preferred Securities Not Carried At Fair Value	433	446
Total Liabilities Not Carried At Fair Value Not Carried At Fair Value	$ 15,672	$ 11,528