Fair Values of Financial Instruments - Fair Values of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	$ 2,284	$ 1,893
Tax Exempt, Fair Value	20,157	20,211
Taxable, fair value	18,060	16,973
Total fixed maturities, Fair Value	38,217	37,184
Equity securities	1,613	1,512
Long Term Debt	3,575	3,575
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	2,284	1,893
Total fixed maturities, Fair Value	38,217	37,184
Equity securities	1,613	1,512
Total fixed maturities and equity securities, Fair value	42,114	40,589
Long Term Debt	4,427	4,085
U.S. government and government agency and authority obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	1,056	868
Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	7,764	6,465
Foreign government and government agency obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	6,949	6,823
Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	527	855
Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	1,764	1,962
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	171	117
Tax Exempt, Fair Value
Taxable, fair value
Total fixed maturities, Fair Value
Equity securities	1,605	1,504
Total fixed maturities and equity securities, Fair value	1,776	1,621
Long Term Debt
Level 1 [Member] | U.S. government and government agency and authority obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 1 [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 1 [Member] | Foreign government and government agency obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 1 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 1 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	2,113	1,776
Tax Exempt, Fair Value	20,151	20,203
Taxable, fair value	17,902	16,808
Total fixed maturities, Fair Value	38,053	37,011
Equity securities
Total fixed maturities and equity securities, Fair value	40,166	38,787
Long Term Debt	4,427	4,085
Level 2 [Member] | U.S. government and government agency and authority obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	1,056	868
Level 2 [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	7,616	6,313
Level 2 [Member] | Foreign government and government agency obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	6,949	6,820
Level 2 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	517	845
Level 2 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	1,764	1,962
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments
Tax Exempt, Fair Value	6	8
Taxable, fair value	158	165
Total fixed maturities, Fair Value	164	173
Equity securities	8	8
Total fixed maturities and equity securities, Fair value	172	181
Long Term Debt
Level 3 [Member] | U.S. government and government agency and authority obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value
Level 3 [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	148	152
Level 3 [Member] | Foreign government and government agency obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value		3
Level 3 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value	10	10
Level 3 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, fair value