Fair Value Measurements (Carrying Values And Fair Values Of Financial Instruments Not Measured At Fair Value) (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	$ 8,805	$ 7,589
Investments in partially-owned insurance companies	638	504
Total assets	9,443	8,093
Short-term debt	2,107	2,571
Long-term debt	4,383	3,690
Trust preferred securities	457	462
Total liabilities	6,947	6,723
Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	8,676	7,331
Investments in partially-owned insurance companies	638	504
Total assets	9,314	7,835
Short-term debt	2,102	2,552
Long-term debt	4,157	3,357
Trust preferred securities	309	309
Total liabilities	6,568	6,218
Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	583	659
Investments in partially-owned insurance companies	0	0
Total assets	583	659
Short-term debt	0	0
Long-term debt	0	0
Trust preferred securities	0	0
Total liabilities	0	0
Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	8,208	6,915
Investments in partially-owned insurance companies	0	0
Total assets	8,208	6,915
Short-term debt	2,107	2,571
Long-term debt	4,383	3,690
Trust preferred securities	457	462
Total liabilities	6,947	6,723
Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	14	15
Investments in partially-owned insurance companies	638	504
Total assets	652	519
Short-term debt	0	0
Long-term debt	0	0
Trust preferred securities	0	0
Total liabilities	0	0
U.S. Treasury And Agency [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	758	850
U.S. Treasury And Agency [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	743	832
U.S. Treasury And Agency [Member] \| Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	583	659
U.S. Treasury And Agency [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	175	191
U.S. Treasury And Agency [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
Foreign [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	843	963
Foreign [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	808	916
Foreign [Member] \| Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
Foreign [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	843	963
Foreign [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
Corporate Debt Securities [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	3,129	2,423
Corporate Debt Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	3,113	2,323
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	3,115	2,408
Corporate Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	14	15
Collateralized Mortgage Backed Securities [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	1,877	2,039
Collateralized Mortgage Backed Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	1,828	1,983
Collateralized Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
Collateralized Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	1,877	2,039
Collateralized Mortgage Backed Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
US States and Political Subdivisions Debt Securities [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	2,198	1,314
US States and Political Subdivisions Debt Securities [Member] \| Carrying Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	2,184	1,277
US States and Political Subdivisions Debt Securities [Member] \| Fair Value, Inputs, Level 1 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	0	0
US States and Political Subdivisions Debt Securities [Member] \| Fair Value, Inputs, Level 2 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	2,198	1,314
US States and Political Subdivisions Debt Securities [Member] \| Fair Value, Inputs, Level 3 [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities held to maturity, at amortized cost (fair value – $8,805 and $7,589)	$ 0	$ 0

Sum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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Total investments in (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership.

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Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

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Sum of the carrying amounts as of the balance sheet date of all liabilities that are recognized. Liabilities are probable future sacrifices of economic benefits arising from present obligations of an entity to transfer assets or provide services to other entities in the future.

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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