Goodwill and other intangible assets - Intangible assets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 40,874	$ 15,498
Accumulated amortization	6,788	5,701
Insurance and Other [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	39,976	14,610
Accumulated amortization	6,495	5,462
Railroad, Utilities and Energy [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	898	888
Accumulated amortization	293	239
Trademarks and trade names [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	5,175	3,041
Accumulated amortization	616	765
Other intangible assets [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	3,115	2,731
Accumulated amortization	965	755
Patents and technology [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	4,341	4,252
Accumulated amortization	2,328	2,050
Customer relationships [Member]
Finite-Lived and Indefinite-Lived Intangible Assets [Line Items]
Gross carrying amount	28,243	5,474
Accumulated amortization	$ 2,879	$ 2,131