Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short Term Investments	$ 1,411	$ 1,318
Fixed maturities, Fair Value	37,880	38,780
Equity securities	1,957	1,964
Long Term Debt	3,300	3,300
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short Term Investments	1,411	1,318
Fixed maturities, Fair Value	37,880	38,780
Equity securities	1,957	1,964
Total fixed maturities, equity securities and other invested assets, Fair Value	41,248	42,062
Long Term Debt	3,864	4,013
Tax Exempt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	20,244	19,772
Tax Exempt [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	20,244	19,772
Taxable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	17,636	19,008
Taxable [Member] \| U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	1,279	2,007
Taxable [Member] \| Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	8,973	9,028
Taxable [Member] \| Foreign Government and Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	6,221	6,672
Taxable [Member] \| Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	17,636	19,008
Taxable [Member] \| Fair Value [Member] \| U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	1,279	2,007
Taxable [Member] \| Fair Value [Member] \| Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	8,973	9,028
Taxable [Member] \| Fair Value [Member] \| Foreign Government and Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	6,221	6,672
Taxable [Member] \| Fair Value [Member] \| Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	174	211
Taxable [Member] \| Fair Value [Member] \| Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	989	1,090
Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short Term Investments	235	206
Equity securities	1,951	1,958
Total fixed maturities, equity securities and other invested assets, Fair Value	2,186	2,164
Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short Term Investments	1,176	1,112
Fixed maturities, Fair Value	37,635	38,651
Total fixed maturities, equity securities and other invested assets, Fair Value	38,811	39,763
Long Term Debt	3,864	4,013
Level 2 [Member] \| Tax Exempt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	20,242	19,769
Level 2 [Member] \| Taxable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	17,393	18,882
Level 2 [Member] \| Taxable [Member] \| U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	1,279	2,007
Level 2 [Member] \| Taxable [Member] \| Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	8,784	8,912
Level 2 [Member] \| Taxable [Member] \| Foreign Government and Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	6,213	6,663
Level 2 [Member] \| Taxable [Member] \| Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	172	210
Level 2 [Member] \| Taxable [Member] \| Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	945	1,090
Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	245	129
Equity securities	6	6
Total fixed maturities, equity securities and other invested assets, Fair Value	251	135
Level 3 [Member] \| Tax Exempt [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	2	3
Level 3 [Member] \| Taxable [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	243	126
Level 3 [Member] \| Taxable [Member] \| Corporate Bonds [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	189	116
Level 3 [Member] \| Taxable [Member] \| Foreign Government and Government Agency Obligations [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	8	9
Level 3 [Member] \| Taxable [Member] \| Residential Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	2	$ 1
Level 3 [Member] \| Taxable [Member] \| Commercial Mortgage-backed Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fixed maturities, Fair Value	$ 44

Amount of debt securities categorized neither as held-to-maturity nor trading.

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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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Total debt and equity financial instruments including: (1) securities held-to-maturity, (2) trading securities, and (3) securities available-for-sale.

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Other investments not otherwise specified in the taxonomy that will be realized in a short period of time, usually less than one year or the normal operating cycle, whichever is longer.

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