Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	$ 1,318	$ 2,114
Fixed maturities, Fair Value	38,780	37,091
Equity securities	1,964	1,810
Long Term Debt	3,300	3,300
Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,318	2,114
Fixed maturities, Fair Value	38,780	37,091
Equity securities	1,964	1,810
Total fixed maturities, equity securities and other invested assets, Fair Value	42,062	41,015
Long Term Debt	4,013	3,806
Tax Exempt [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	19,772	18,421
Tax Exempt [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	19,772	18,421
Taxable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	19,008	18,670
Taxable [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	2,007	802
Taxable [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	9,028	9,314
Taxable [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	6,672	6,890
Taxable [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	19,008	18,670
Taxable [Member] | Fair Value [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	2,007	802
Taxable [Member] | Fair Value [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	9,028	9,314
Taxable [Member] | Fair Value [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	6,672	6,890
Taxable [Member] | Fair Value [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	211	299
Taxable [Member] | Fair Value [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	1,090	1,365
Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	206	399
Fixed maturities, Fair Value
Equity securities	1,958	1,803
Total fixed maturities, equity securities and other invested assets, Fair Value	2,164	2,202
Long Term Debt
Level 1 [Member] | Tax Exempt [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 1 [Member] | Taxable [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,112	1,715
Fixed maturities, Fair Value	38,651	36,916
Equity securities
Total fixed maturities, equity securities and other invested assets, Fair Value	39,763	38,631
Long Term Debt	4,013	3,806
Level 2 [Member] | Tax Exempt [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	19,769	18,416
Level 2 [Member] | Taxable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	18,882	18,500
Level 2 [Member] | Taxable [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	2,007	802
Level 2 [Member] | Taxable [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	8,912	9,179
Level 2 [Member] | Taxable [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	6,663	6,881
Level 2 [Member] | Taxable [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	210	293
Level 2 [Member] | Taxable [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	1,090	1,345
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments
Fixed maturities, Fair Value	129	175
Equity securities	6	7
Total fixed maturities, equity securities and other invested assets, Fair Value	135	182
Long Term Debt
Level 3 [Member] | Tax Exempt [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	3	5
Level 3 [Member] | Taxable [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	126	170
Level 3 [Member] | Taxable [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value
Level 3 [Member] | Taxable [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	116	135
Level 3 [Member] | Taxable [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	9	9
Level 3 [Member] | Taxable [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value	1	6
Level 3 [Member] | Taxable [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Fixed maturities, Fair Value		$ 20