Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 50,874	$ 50,831
Accumulated amortization	16,236	14,947
Net carrying value	34,638	35,884
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,990	2,015
Accumulated amortization	854	759
Net carrying value	1,136	1,256
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	9,007	8,997
Accumulated amortization	1,041	899
Net carrying value	7,966	8,098
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	30,941	30,832
Accumulated amortization	8,840	8,048
Net carrying value	22,101	22,784
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,551	5,764
Accumulated amortization	1,996	1,891
Net carrying value	3,555	3,873
Other intangible assets [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	437	450
Accumulated amortization	126	115
Net carrying value	311	335
Customer relationships and contracts [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,553	1,565
Accumulated amortization	728	644
Net carrying value	825	921
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,375	5,238
Accumulated amortization	4,359	4,109
Net carrying value	$ 1,016	$ 1,129