Business segment data - Narrative (Detail) - USD ($) $ in Billions		12 Months Ended
	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Sales and service revenues		$ 112.4	$ 102.2	$ 97.6
Wal-Mart Stores, Inc. [Member]
Segment Reporting Information [Line Items]
Sales and service revenues		$ 13.0	$ 13.0	$ 13.0
Sales and Service Revenues [Member] | Geographic Concentration [Member] | United States [Member]
Segment Reporting Information [Line Items]
Concentration percentage		87.00%	85.00%	85.00%
Property, Plant and Equipment [Member] | Geographic Concentration [Member] | United States [Member]
Segment Reporting Information [Line Items]
Concentration percentage	89.00%
Railroad, Utilities and Energy [Member] | Revenues [Member] | Geographic Concentration [Member] | United States [Member]
Segment Reporting Information [Line Items]
Concentration percentage		95.00%	96.00%	96.00%