Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 41,153	$ 40,493
Accumulated amortization	10,102	8,994
Insurance and Other [Member] | Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,286	5,152
Accumulated amortization	759	727
Insurance and Other [Member] | Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	27,943	27,697
Accumulated amortization	5,025	4,287
Insurance and Other [Member] | Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,364	3,198
Accumulated amortization	1,286	1,190
Insurance and Other [Member] | Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,560	4,446
Accumulated amortization	3,032	2,790
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	1,003	1,011
Accumulated amortization	408	362
Railroad, Utilities and Energy [Member] | Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	212	216
Accumulated amortization	26	23
Railroad, Utilities and Energy [Member] | Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	678	678
Accumulated amortization	324	286
Railroad, Utilities and Energy [Member] | Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	113	117
Accumulated amortization	$ 58	$ 53