Investments in equity securities - Fair value by segment (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 118,486		$ 117,505
Insurance and Other [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	116,160		115,464
Railroad, Utilities and Energy [Member] | Other Assets [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	1,391	[1]	1,103	[1]
Finance and Financial Products [Member]
Summary of Investment Holdings [Line Items]
Investments in equity securities	$ 935		$ 938

[1] Included in other assets.