Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 40,576	$ 41,153
Accumulated amortization	10,924	10,102
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,003	1,003
Accumulated amortization	443	408
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,193	5,286
Accumulated amortization	774	759
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	27,344	27,943
Accumulated amortization	5,548	5,025
Customer relationships [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	678	678
Accumulated amortization	352	324
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,319	3,364
Accumulated amortization	1,347	1,286
Trademarks, Trade Names and Other [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	325	325
Accumulated amortization	91	84
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,720	4,560
Accumulated amortization	$ 3,255	$ 3,032