Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 76,991		$ 61,513
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	76,063		59,819
Railroad, Utilities and Energy [Member] | Other Asset [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	488	[1]	1,182	[1]
Finance and Financial Products [Member] | Other Asset [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 440	[1]	$ 512	[1]

[1] Included in other assets.