Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 40,685	$ 41,153
Accumulated amortization	11,223	10,102
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,681	1,003
Accumulated amortization	459	408
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,206	5,286
Accumulated amortization	779	759
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	27,374	27,943
Accumulated amortization	5,756	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	361	324
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,339	3,364
Accumulated amortization	1,375	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	1,003	325
Accumulated amortization	98	84
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,766	4,560
Accumulated amortization	$ 3,313	$ 3,032