Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	$ 1,693	$ 2,114
Tax exempt, Fair Value	18,784	18,421
Taxable, Fair Value	19,041	18,670
Total fixed maturities, Fair Value	37,825	37,091
Equity securities	1,863	1,810
Long Term Debt	3,300	3,300
Residential Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	273	299
Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,258	1,365
U.S. Government and Government Agency and Authority Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	805	802
Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,820	9,314
Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,885	6,890
Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,693	2,114
Tax exempt, Fair Value	18,784	18,421
Taxable, Fair Value	19,041	18,670
Total fixed maturities, Fair Value	37,825	37,091
Equity securities	1,863	1,810
Total fixed maturities, equity securities and other invested assets, Fair Value	41,381	41,015
Long Term Debt	3,922	3,806
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	805	802
Fair value [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,820	9,314
Fair value [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,885	6,890
Residential Mortgage-backed Securities [Member] | Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	273	299
Commercial Mortgage-backed Securities [Member] | Fair value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,258	1,365
Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	256	399
Tax exempt, Fair Value
Taxable, Fair Value
Total fixed maturities, Fair Value
Equity securities	1,857	1,803
Total fixed maturities, equity securities and other invested assets, Fair Value	2,113	2,202
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	1,437	1,715
Tax exempt, Fair Value	18,780	18,416
Taxable, Fair Value	18,892	18,500
Total fixed maturities, Fair Value	37,672	36,916
Equity securities
Total fixed maturities, equity securities and other invested assets, Fair Value	39,109	38,631
Long Term Debt	3,922	3,806
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	805	802
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,690	9,179
Level 2 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,871	6,881
Level 2 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	268	293
Level 2 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,258	1,345
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments
Tax exempt, Fair Value	4	5
Taxable, Fair Value	149	170
Total fixed maturities, Fair Value	153	175
Equity securities	6	7
Total fixed maturities, equity securities and other invested assets, Fair Value	159	182
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	130	135
Level 3 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	14	9
Level 3 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	5	6
Level 3 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value		$ 20