Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 50,917	$ 50,874
Accumulated amortization	16,586	16,236
Net carrying value	34,331	34,638
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,977	1,990
Accumulated amortization	867	854
Net carrying value	1,110	1,136
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	30,896	30,941
Accumulated amortization	8,990	8,840
Net carrying value	21,906	22,101
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	9,017	9,007
Accumulated amortization	1,075	1,041
Net carrying value	7,942	7,966
Customer relationships and contracts [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,541	1,553
Accumulated amortization	738	728
Net carrying value	803	825
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,571	5,551
Accumulated amortization	2,071	1,996
Net carrying value	3,500	3,555
Other intangible assets [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	436	437
Accumulated amortization	129	126
Net carrying value	307	311
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,433	5,375
Accumulated amortization	4,450	4,359
Net carrying value	$ 983	$ 1,016