Supplemental cash flow information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for income taxes	$ 3,286	$ 4,719	$ 4,535
Liabilities assumed in connection with business acquisitions	747	16,555	2,812
Equity securities exchanged in connection with business acquisitions		4,239
Conversions and other exchanges of investments		4,154	1,597
Equity securities surrendered in connection with warrant exercise	4,965
Insurance and Other [Member]
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for interest	871	555	346
Railroad, Utilities and Energy [Member]
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for interest	2,828	2,788	2,717
Finance and Financial Products [Member]
Supplemental Cash Flow Information [Line Items]
Cash paid during the period for interest	$ 389	$ 389	$ 403