Valuation and qualifying accounts (Tables)

12 Months Ended

Dec. 31, 2017

Valuation and Qualifying Accounts [Abstract]

Summary of valuation and qualifying accounts

											$ million
			2017				2016				2015
	Accounts receivable		Fixed asset investments		Accounts receivable		Fixed asset investments		Accounts receivable		Fixed asset investments
At 1 January	392		335		447		435		331		517
Charged to costs and expenses	68		47		120		55		243		195
Charged to other accountsa	13		3		(7	)	(2	)	(23	)	(4	)
Deductions	(138	)	(71	)	(168	)	(153	)	(104	)	(273	)
At 31 December	335		314		392		335		447		435

^a  Principally exchange adjustments.