Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012		Dec. 31, 2011
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 89,115		$ 76,991
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	87,993		76,063
Railroad, Utilities and Energy [Member] | Other Asset [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	629	[1]	488	[1]
Finance and Financial Products [Member] | Other Asset [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 493	[1]	$ 440	[1]

[1] Included in other assets.