Business segment data - Narrative (Detail) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenues		$ 63,714	$ 63,450	$ 62,200	$ 58,473	$ 58,800	$ 59,507	$ 57,256	$ 64,370	$ 247,837	$ 239,933	$ 215,114
Sales, Service and Leasing Revenues [Member]
Segment Reporting Information [Line Items]
Revenues										$ 139,100	$ 132,800	$ 125,600
United States [Member] | Geographic Concentration [Member] | Sales, Service and Leasing Revenues [Member]
Segment Reporting Information [Line Items]
Concentration percentage										84.00%	85.00%	85.00%
United States [Member] | Geographic Concentration [Member] | Property, Plant and Equipment and Equipment Held for Lease [Member]
Segment Reporting Information [Line Items]
Concentration percentage	89.00%
Walmart Stores, Inc. [Member] | Sales, Service and Leasing Revenues [Member]
Segment Reporting Information [Line Items]
Revenues										$ 13,000	$ 14,000	$ 14,000
Railroad, Utilities and Energy [Member]
Segment Reporting Information [Line Items]
Revenues										$ 43,673	$ 40,005	$ 37,447
Railroad, Utilities and Energy [Member] | United States [Member] | Geographic Concentration [Member] | Revenues [Member]
Segment Reporting Information [Line Items]
Concentration percentage										96.00%	95.00%	95.00%