Restructuring and other cost savings initiatives (Summary of Expenses and Payments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Restructuring Reserve [Roll Forward]
Restructuring liabilities as of beginning of period	$ 20	$ 118	$ 244	$ 0
Expense	79	19	132	385
Payments	(31)	(117)	(258)	(141)
Restructuring liabilities as of end of period	68	20	118	244
Separation costs [Member]
Restructuring Reserve [Roll Forward]
Restructuring liabilities as of beginning of period	11	95	221	0
Expense	72	6	52	353
Payments	(20)	(90)	(178)	(132)
Restructuring liabilities as of end of period	63	11	95	221
Other [Member]
Restructuring Reserve [Roll Forward]
Restructuring liabilities as of beginning of period	9	23	23	0
Expense	7	13	80	32
Payments	(11)	(27)	(80)	(9)
Restructuring liabilities as of end of period	$ 5	$ 9	$ 23	$ 23