Business segment data - Narrative (Detail) - USD ($) $ in Billions		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Sales and service revenues		$ 132.9	$ 125.7	$ 112.4
United States [Member] | Geographic Concentration [Member] | Sales and Service Revenues [Member]
Segment Reporting Information [Line Items]
Concentration percentage		85.00%	85.00%	87.00%
United States [Member] | Geographic Concentration [Member] | Property, Plant and Equipment [Member]
Segment Reporting Information [Line Items]
Concentration percentage	89.00%
Walmart Stores, Inc. [Member]
Segment Reporting Information [Line Items]
Sales and service revenues		$ 14.0	$ 14.0	$ 13.0
Railroad, Utilities and Energy [Member] | United States [Member] | Geographic Concentration [Member] | Revenues [Member]
Segment Reporting Information [Line Items]
Concentration percentage		95.00%	95.00%	95.00%