Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 44,361	$ 43,988
Accumulated amortization	15,564	14,661
Net carrying value	28,797	29,327
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	8,840	8,858
Accumulated amortization	1,393	1,045
Net carrying value	7,447	7,813
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,496	28,305
Accumulated amortization	8,406	7,901
Net carrying value	20,090	20,404
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,686	5,619
Accumulated amortization	866	846
Net carrying value	4,820	4,773
Trademarks and trade names [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,591	3,592
Accumulated amortization	190	98
Net carrying value	3,401	3,494
Customer relationships and contracts [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,080	4,092
Accumulated amortization	972	791
Net carrying value	3,108	3,301
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,773	4,826
Accumulated amortization	1,910	1,805
Net carrying value	2,863	3,021
Other intangible assets [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,169	1,174
Accumulated amortization	231	156
Net carrying value	938	1,018
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,406	5,238
Accumulated amortization	4,382	4,109
Net carrying value	$ 1,024	$ 1,129