Significant accounting policies and practices - Narrative (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Voting interests of investee	20.00%
Percentage of LIFO Inventory	35.00%
Excess of current or replacement costs over costs determined under LIFO	$ 1,100		$ 950
Retained earnings	444,626		402,493
Operating leases, right of use assets	5,579		5,941
Operating lease liabilities	5,469		5,882
Assets	873,729		817,729		$ 707,794
Liabilities	$ 422,393		389,166
Difference Between Lease Guidance in Effect Before and After Topic 842 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating leases, right of use assets				$ 6,200
Operating lease liabilities				5,900
ASU 2016-13 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2020
ASU 2016-13 [Member] | Cumulative effect, period of adoption, adjustment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings		$ (388)
ASC 842 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2019
ASC 842 [Member] | Difference Between Lease Guidance in Effect Before and After Topic 842 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other assets				(300)
Assets				5,900
Liabilities				$ 5,900
ASU 2016-01 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Retained earnings						$ 61,459
Change in accounting principle, accounting standards update, adopted	true
Change in accounting principle, accounting standards update, adoption date	Jan. 01, 2018
Other Assets [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Unamortized balances of deferred policy acquisition costs	$ 3,250		$ 2,950
Maximum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Trade receivables, insurance premium receivables and other receivables collection term	1 year
Insurance interest rate assumption	7.00%
Maximum [Member] | Buildings and improvements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	50 years
Maximum [Member] | Machinery and equipment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	25 years
Maximum [Member] | Furniture, fixtures and other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	15 years
Maximum [Member] | Track structure and other roadway [Member] | Railroad [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	100 years
Maximum [Member] | Locomotives, freight cars and other equipment [Member] | Railroad [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	43 years
Maximum [Member] | Utility generation, transmission and distribution systems [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	80 years
Maximum [Member] | Interstate natural gas pipeline assets [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	80 years
Maximum [Member] | Independent power plants and other assets [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	40 years
Maximum [Member] | Equipment held for lease [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	35 years
Minimum [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Insurance interest rate assumption	3.00%
Minimum [Member] | Buildings and improvements [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	5 years
Minimum [Member] | Machinery and equipment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	3 years
Minimum [Member] | Furniture, fixtures and other [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	3 years
Minimum [Member] | Track structure and other roadway [Member] | Railroad [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	10 years
Minimum [Member] | Locomotives, freight cars and other equipment [Member] | Railroad [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	6 years
Minimum [Member] | Utility generation, transmission and distribution systems [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	5 years
Minimum [Member] | Interstate natural gas pipeline assets [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	3 years
Minimum [Member] | Independent power plants and other assets [Member] | Utilities and Energy [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	3 years
Minimum [Member] | Equipment held for lease [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Estimated useful life (in years)	3 years