Goodwill and other intangible assets - Other intangible assets and related accumulated amortization (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 41,213	$ 40,874
Accumulated amortization	8,031	6,788
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	40,225	39,976
Accumulated amortization	7,707	6,495
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	988	898
Accumulated amortization	324	293
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,381	5,175
Accumulated amortization	692	616
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,322	28,243
Accumulated amortization	3,722	2,879
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,169	3,115
Accumulated amortization	1,124	965
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,341	4,341
Accumulated amortization	$ 2,493	$ 2,328