Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 41,502	$ 40,874
Accumulated amortization	7,877	6,788
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	40,528	39,976
Accumulated amortization	7,554	6,495
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	974	898
Accumulated amortization	323	293
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,352	5,175
Accumulated amortization	672	616
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,166	3,115
Accumulated amortization	1,083	965
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,487	4,341
Accumulated amortization	2,605	2,328
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,497	28,243
Accumulated amortization	$ 3,517	$ 2,879