Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 103,274		$ 87,662
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	101,933		86,467
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	745	[1]	675	[1]
Finance and Financial Products [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 596	[1]	$ 520	[1]

[1] Included in other assets.