Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 41,624	$ 41,504
Accumulated amortization	9,966	9,356
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	40,613	40,493
Accumulated amortization	9,581	8,994
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,011	1,011
Accumulated amortization	385	362
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,368	5,368
Accumulated amortization	763	750
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,449	28,375
Accumulated amortization	4,990	4,573
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,305	3,315
Accumulated amortization	1,286	1,243
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,502	4,446
Accumulated amortization	$ 2,927	$ 2,790