Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 21	$ 26	$ 61	$ 72
Interest cost	33	34	99	104
Expected return on plan assets	(58)	(52)	(173)	(151)
Amortization of net actuarial loss	1	0	2	2
Amortization of prior service cost	0		0
Curtailments	0	(4)	(8)	(4)
Settlements		(1)		(1)
Net periodic (benefit) cost	(3)	3	(19)	22
Pension Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	15	20	47	57
Interest cost	27	26	79	80
Expected return on plan assets	(47)	(42)	(142)	(121)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	0		0
Curtailments	0	(4)	0	(4)
Settlements		(1)		(1)
Net periodic (benefit) cost	(5)	(1)	(16)	11
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	6	6	14	15
Interest cost	6	8	20	24
Expected return on plan assets	(11)	(10)	(31)	(30)
Amortization of net actuarial loss	1	0	2	2
Amortization of prior service cost	0		0
Curtailments	0	0	(8)	0
Settlements		0		0
Net periodic (benefit) cost	2	4	(3)	11
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	4	2	9
Interest cost	2	4	3	13
Expected return on plan assets	(2)	(3)	(4)	(7)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	(22)		(68)
Curtailments	(32)	0	(32)	0
Settlements		0		0
Net periodic (benefit) cost	(54)	5	(99)	15
Other Postretirement Benefits Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	4	1	8
Interest cost	1	4	2	13
Expected return on plan assets	(2)	(3)	(4)	(7)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	(22)		(68)
Curtailments	(32)	0	(32)	0
Settlements		0		0
Net periodic (benefit) cost	(55)	5	(101)	14
Other Postretirement Benefits Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	0	1	1
Interest cost	1	0	1	0
Expected return on plan assets	0	0	0	0
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	0		0
Curtailments	0	0	0	0
Settlements		0		0
Net periodic (benefit) cost	$ 1	$ 0	$ 2	$ 1