Fair value measurements - Financial assets and liabilities measured and carried at fair value on a recurring basis (Detail) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	$ 34,048		$ 34,883
Investments in equity securities	68,062		61,513
Other investments	15,276		17,589
Railroad, Utilities and Energy [Member] | Quoted Prices (Level 1) [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	18		7
Quoted Prices (Level 1) [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in equity securities	67,964		61,390
Quoted Prices (Level 1) [Member] | U.S. Treasury, U.S. government corporations and agencies [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	686		535
Quoted Prices (Level 1) [Member] | Foreign governments [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	5,765		5,633
Quoted Prices (Level 1) [Member] | Corporate bonds [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities			23
Railroad, Utilities and Energy [Member] | Significant Other Observable Inputs (Level 2) [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	188		52
Finance and Financial Products [Member] | Significant Other Observable Inputs (Level 2) [Member]
Net derivative contract (assets)/liabilities:
Other	26		137
Significant Other Observable Inputs (Level 2) [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in equity securities	82		88
Other investments	3,300
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury, U.S. government corporations and agencies [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	1,603		1,658
Significant Other Observable Inputs (Level 2) [Member] | States, municipalities and political subdivisions [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	3,153		3,581
Significant Other Observable Inputs (Level 2) [Member] | Foreign governments [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	7,309		6,167
Significant Other Observable Inputs (Level 2) [Member] | Corporate bonds [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	11,947		13,346
Significant Other Observable Inputs (Level 2) [Member] | Mortgage-backed securities [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	2,805		3,139
Railroad, Utilities and Energy [Member] | Significant Unobservable Inputs (Level 3) [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	207		331
Finance and Financial Products [Member] | Significant Unobservable Inputs (Level 3) [Member]
Net derivative contract (assets)/liabilities:
Equity index put options	8,849		6,712
Credit default obligations	1,324		1,239
Other	23		(60)
Significant Unobservable Inputs (Level 3) [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in equity securities	16		35
Other investments	11,976		17,589
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury, U.S. government corporations and agencies [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	3		4
Significant Unobservable Inputs (Level 3) [Member] | States, municipalities and political subdivisions [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	1
Significant Unobservable Inputs (Level 3) [Member] | Foreign governments [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	122		112
Significant Unobservable Inputs (Level 3) [Member] | Corporate bonds [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	654		685
U.S. Treasury, U.S. government corporations and agencies [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	2,292		2,197
States, municipalities and political subdivisions [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	3,154		3,581
Foreign governments [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	13,196		11,912
Corporate bonds [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	12,601		14,054
Mortgage-backed securities [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	2,805		3,139
Railroad, Utilities and Energy [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in equity securities	381	[1]	1,182	[1]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	413		390
Finance and Financial Products [Member]
Fair value measurements Financial assets and liabilities measured and carried at fair value on a recurring basis [Line Items]
Investments in fixed maturity securities	1,017		1,080
Investments in equity securities	456	[1]	512	[1]
Net derivative contract (assets)/liabilities:
Equity index put options	8,849		6,712
Credit default obligations	1,324		1,239
Other	$ 49		$ 77

[1] Included in Other assets.