Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 41,322	$ 40,874
Accumulated amortization	7,511	6,788
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	40,419	39,976
Accumulated amortization	7,199	6,495
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	903	898
Accumulated amortization	312	293
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,275	5,175
Accumulated amortization	653	616
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,155	3,115
Accumulated amortization	1,047	965
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,435	4,341
Accumulated amortization	2,512	2,328
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,457	28,243
Accumulated amortization	$ 3,299	$ 2,879