Supplemental cash flow information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for income taxes	$ 5,001	$ 5,415	$ 4,354
Non-cash investing and financing activities:
Liabilities assumed in connection with business acquisitions	6,981	766	3,735
Operating lease liabilities arising from obtaining right-of-use assets	729	782
Insurance and Other [Member]
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for interest	1,001	1,011	1,111
Railroad, Utilities and Energy [Member]
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for interest	$ 3,006	$ 2,879	$ 2,867