Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	$ 1,845	$ 2,114
Tax exempt, Fair Value	19,220	18,421
Taxable, Fair Value	18,842	18,670
Total fixed maturities, Fair Value	38,062	37,091
Equity securities	1,969	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	250	299
Commercial Mortgage Backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,207	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	1,286	802
Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,194	9,314
Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	6,905	6,890
Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,845	2,114
Tax exempt, Fair Value	19,220	18,421
Taxable, Fair Value	18,842	18,670
Total fixed maturities, Fair Value	38,062	37,091
Equity securities	1,969	1,810
Total fixed maturities, equity securities and other invested assets, Fair Value	41,876	41,015
Long Term Debt	3,989	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	1,286	802
Fair Value [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,194	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	6,905	6,890
Residential Mortgage-backed Securities [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	250	299
Commercial Mortgage-backed Securities [Member] | Fair Value [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,207	1,365
Level 1 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	284	399
Equity securities	1,963	1,803
Total fixed maturities, equity securities and other invested assets, Fair Value	2,247	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 2 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Short Term Investments	1,561	1,715
Tax exempt, Fair Value	19,216	18,416
Taxable, Fair Value	18,690	18,500
Total fixed maturities, Fair Value	37,906	36,916
Total fixed maturities, equity securities and other invested assets, Fair Value	39,467	38,631
Long Term Debt	3,989	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	1,286	802
Level 2 [Member] | Corporate Bonds [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9,066	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	6,896	6,881
Level 2 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	246	293
Level 2 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	1,196	1,345
Level 3 [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Tax exempt, Fair Value	4	5
Taxable, Fair Value	152	170
Total fixed maturities, Fair Value	156	175
Equity securities	6	7
Total fixed maturities, equity securities and other invested assets, Fair Value	162	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	128	135
Level 3 [Member] | Foreign Government and Government Agency Obligations [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	9	9
Level 3 [Member] | Residential Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	4	6
Level 3 [Member] | Commercial Mortgage-backed Securities [Member]
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Taxable, Fair Value	$ 11	$ 20