Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 40,601	$ 40,685
Accumulated amortization	11,417	11,223
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,681	1,681
Accumulated amortization	480	459
Trademarks and trade names [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,197	5,206
Accumulated amortization	784	779
Customer relationships [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	27,353	27,374
Accumulated amortization	5,927	5,756
Customer relationships [Member] | Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	678	678
Accumulated amortization	370	361
Other intangible assets [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,322	3,339
Accumulated amortization	1,390	1,375
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	1,003
Accumulated amortization	110	98
Patents and technology [Member] | Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,729	4,766
Accumulated amortization	$ 3,316	$ 3,313