Fair value measurements - Financial assets and liabilities measured and carried at fair value on a recurring basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	$ 76,991	$ 61,513
Finance and Financial Products [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	966	1,080
Fixed Maturities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	32,188	34,883
Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,935	2,197
Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	3,070	3,581
Fixed Maturities [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	10,843	11,912
Fixed Maturities [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	12,448	14,054
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,892	3,139
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	76,991	61,513
Other investments	11,669	17,589
Fair Value, Measurements, Recurring [Member] | Railroad, Utilities and Energy [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	265	390
Fair Value, Measurements, Recurring [Member] | Finance and Financial Products [Member] | Equity Index Put Options [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	8,499	6,712
Fair Value, Measurements, Recurring [Member] | Finance and Financial Products [Member] | Credit Default Contracts [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	1,472	1,239
Fair Value, Measurements, Recurring [Member] | Finance and Financial Products [Member] | Other Derivative [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	(88)	77
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,935	2,197
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	3,070	3,581
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	10,843	11,912
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	12,448	14,054
Fair Value, Measurements, Recurring [Member] | Fixed Maturities [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,892	3,139
Fair Value, Measurements, Recurring [Member] | Quoted Prices (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	76,906	61,390
Fair Value, Measurements, Recurring [Member] | Quoted Prices (Level 1) [Member] | Railroad, Utilities and Energy [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	12	7
Fair Value, Measurements, Recurring [Member] | Quoted Prices (Level 1) [Member] | Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	843	535
Fair Value, Measurements, Recurring [Member] | Quoted Prices (Level 1) [Member] | Fixed Maturities [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	4,444	5,633
Fair Value, Measurements, Recurring [Member] | Quoted Prices (Level 1) [Member] | Fixed Maturities [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities		23
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	63	88
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Railroad, Utilities and Energy [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	276	52
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Finance and Financial Products [Member] | Other Derivative [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	(48)	137
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,090	1,658
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	3,069	3,581
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	6,265	6,167
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	11,801	13,346
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Fixed Maturities [Member] | Mortgage-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2,892	3,139
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in equity securities	22	35
Other investments	11,669	17,589
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Railroad, Utilities and Energy [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	(23)	331
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Finance and Financial Products [Member] | Equity Index Put Options [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	8,499	6,712
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Finance and Financial Products [Member] | Credit Default Contracts [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	1,472	1,239
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Finance and Financial Products [Member] | Other Derivative [Member]
Net derivative contract (assets)/liabilities:
Net derivative contract (assets)/liabilities	(40)	(60)
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturities [Member] | US Government and Government Agencies and Authorities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	2	4
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturities [Member] | States, municipalities and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	1
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturities [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	134	112
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Fixed Maturities [Member] | Corporate bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments in fixed maturity securities	$ 647	$ 685