Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 42,472	$ 15,498
Accumulated amortization	6,820	5,701
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	6,049	3,041
Accumulated amortization	821	765
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,455	4,252
Accumulated amortization	2,329	2,050
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,851	5,474
Accumulated amortization	2,721	2,131
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,117	2,731
Accumulated amortization	949	755
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	41,575	14,610
Accumulated amortization	6,541	5,462
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	897	888
Accumulated amortization	$ 279	$ 239