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, 2013	Dec. 31, 2012
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	$ 2,114	$ 2,528
Tax exempt, Fair Value	18,421	19,913
Taxable, Fair Value	18,670	18,163
Total fixed maturities, Fair Value	37,091	38,076
Equity securities	1,810	1,663
Other Invested Assets	1,598	1,954
Long Term Debt	3,300	3,575
Residential Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	299	455
Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,365	1,724
U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	802	1,039
Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,314	7,937
Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,890	7,008
Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	2,114	2,528
Tax exempt, Fair Value	18,421	19,913
Taxable, Fair Value	18,670	18,163
Total fixed maturities, Fair Value	37,091	38,076
Equity securities	1,810	1,663
Other Invested Assets		46
Total fixed maturities, equity securities and other invested assets, Fair Value	41,015	42,313
Long Term Debt	3,806	4,372
Fair value [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	802	1,039
Fair value [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,314	7,937
Fair value [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,890	7,008
Residential Mortgage-backed Securities [Member] | Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	299	455
Commercial Mortgage-backed Securities [Member] | Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,365	1,724
Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	399	182
Tax exempt, Fair Value
Taxable, Fair Value
Total fixed maturities, Fair Value
Equity securities	1,803	1,655
Other Invested Assets
Total fixed maturities, equity securities and other invested assets, Fair Value	2,202	1,837
Long Term Debt
Level 1 [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 1 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 1 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 1 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 1 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,715	2,346
Tax exempt, Fair Value	18,416	19,907
Taxable, Fair Value	18,500	17,996
Total fixed maturities, Fair Value	36,916	37,903
Equity securities
Other Invested Assets
Total fixed maturities, equity securities and other invested assets, Fair Value	38,631	40,249
Long Term Debt	3,806	4,372
Level 2 [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	802	1,039
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,179	7,779
Level 2 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,881	7,008
Level 2 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	293	446
Level 2 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,345	1,724
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments
Tax exempt, Fair Value	5	6
Taxable, Fair Value	170	167
Total fixed maturities, Fair Value	175	173
Equity securities	7	8
Other Invested Assets		46
Total fixed maturities, equity securities and other invested assets, Fair Value	182	227
Long Term Debt
Level 3 [Member] | U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value
Level 3 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	135	158
Level 3 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9
Level 3 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6	9
Level 3 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	$ 20