Fair Values of Financial Instruments - Fair Value of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	$ 1,881	$ 2,528
Tax exempt, Fair Value	18,951	19,913
Taxable, Fair Value	18,659	18,163
Total fixed maturities, Fair Value	37,610	38,076
Equity securities	1,821	1,663
Other invested assets	1,665	1,954
Long Term Debt	3,300	3,575
Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	1,881	2,528
Tax exempt, Fair Value	18,951	19,913
Taxable, Fair Value	18,659	18,163
Total fixed maturities, Fair Value	37,610	38,076
Equity securities	1,821	1,663
Other invested assets		46
Total fixed maturities, equity securities and other invested assets, Fair value	41,312	42,313
Long Term Debt	3,834	4,372
U.S. government and government agency and authority obligations [Member] | Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	818	1,039
Corporate bonds [Member] | Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	9,160	7,937
Foreign government and government agency obligations [Member] | Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	6,987	7,008
Residential mortgage-backed securities [Member] | Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	324	455
Commercial mortgage-backed securities [Member] | Fair value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	1,370	1,724
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments	209	182
Tax exempt, Fair Value
Taxable, Fair Value
Total fixed maturities, Fair Value
Equity securities	1,814	1,655
Other invested assets
Total fixed maturities, equity securities and other invested assets, Fair value	2,023	1,837
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	1,672	2,346
Tax exempt, Fair Value	18,946	19,907
Taxable, Fair Value	18,471	17,996
Total fixed maturities, Fair Value	37,417	37,903
Equity securities
Other invested assets
Total fixed maturities, equity securities and other invested assets, Fair value	39,089	40,249
Long Term Debt	3,834	4,372
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	818	1,039
Level 2 [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	9,031	7,779
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	6,977	7,008
Level 2 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	318	446
Level 2 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	1,327	1,724
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short term investments
Tax exempt, Fair Value	5	6
Taxable, Fair Value	188	167
Total fixed maturities, Fair Value	193	173
Equity securities	7	8
Other invested assets		46
Total fixed maturities, equity securities and other invested assets, Fair value	200	227
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	129	158
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	10
Level 3 [Member] | Residential mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	6	9
Level 3 [Member] | Commercial mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair Value	$ 43