Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 15	$ 16	$ 29	$ 32
Interest cost	27	26	53	52
Expected return on plan assets	(53)	(48)	(106)	(95)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	0	0	0	0
Curtailments	0	0	0	0
Settlement	1	0	1	0
Net periodic (benefit) cost	(10)	(6)	(23)	(11)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	3	4	6	8
Interest cost	7	7	14	14
Expected return on plan assets	(13)	(10)	(26)	(20)
Amortization of net actuarial loss	0	1	0	1
Amortization of prior service cost	0	0	0	0
Curtailments	0	(8)	0	(8)
Settlement	0	0	0	0
Net periodic (benefit) cost	(3)	(6)	(6)	(5)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	0	2	0	2
Interest cost	1	0	2	1
Expected return on plan assets	(1)	(1)	(2)	(2)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	(22)	(23)	(43)	(46)
Curtailments	(1)	0	(1)	0
Settlement	0	0	0	0
Net periodic (benefit) cost	$ (23)	$ (22)	$ (44)	$ (45)