Fair Values of Financial Instruments - Fair Values of Financial Instruments Categorized Based Upon Lowest Level of Input (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments	$ 2,528	$ 1,893
Tax exempt, Fair Value	19,913	20,211
Taxable, Fair value	18,163	16,973
Total fixed maturities, Fair Value	38,076	37,184
Equity securities	1,663	1,512
Other Invested Assets	1,954	2,180
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	2,528	1,893
Tax exempt, Fair Value	19,913	20,211
Taxable, Fair value	18,163	16,973
Total fixed maturities, Fair Value	38,076	37,184
Equity securities	1,663	1,512
Other Invested Assets	46	27
Total fixed maturities, equity securities and other invested assets, Fair Value	42,313	40,616
Long Term Debt	4,372	4,085
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	1,039	868
Corporate bonds [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair value	7,937	6,465
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	7,008	6,823
Residential mortgage-backed securities [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair value	455	855
Commercial mortgage-backed securities [Member] | Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair value	1,724	1,962
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Short Term Investments	182	117
Tax exempt, Fair Value
Taxable, Fair value
Total fixed maturities, Fair Value
Equity securities	1,655	1,504
Other Invested Assets
Total fixed maturities, equity securities and other invested assets, Fair Value	1,837	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	2,346	1,776
Tax exempt, Fair Value	19,907	20,203
Taxable, Fair value	17,996	16,808
Total fixed maturities, Fair Value	37,903	37,011
Equity securities
Other Invested Assets
Total fixed maturities, equity securities and other invested assets, Fair Value	40,249	38,787
Long Term Debt	4,372	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	1,039	868
Level 2 [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Taxable, Fair value	7,779	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	7,008	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	446	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	1,724	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	167	165
Total fixed maturities, Fair Value	173	173
Equity securities	8	8
Other Invested Assets	46	27
Total fixed maturities, equity securities and other invested assets, Fair Value	227	208
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	158	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	9	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