Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 20	$ 24	$ 40	$ 46
Interest cost	33	35	66	70
Expected return on plan assets	(58)	(52)	(115)	(99)
Amortization of net actuarial loss	1	1	1	2
Amortization of prior service cost	0		0
Defined Benefit Plan, Curtailments	(8)		(8)
Net periodic (benefit) cost	(12)	8	(16)	19
Pension Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	16	20	32	37
Interest cost	26	27	52	54
Expected return on plan assets	(48)	(42)	(95)	(79)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	0		0
Defined Benefit Plan, Curtailments	0		0
Net periodic (benefit) cost	(6)	5	(11)	12
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	4	4	8	9
Interest cost	7	8	14	16
Expected return on plan assets	(10)	(10)	(20)	(20)
Amortization of net actuarial loss	1	1	1	2
Amortization of prior service cost	0		0
Defined Benefit Plan, Curtailments	(8)		(8)
Net periodic (benefit) cost	(6)	3	(5)	7
Other Postretirement Benefit Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	2	3	2	5
Interest cost	0	4	1	9
Expected return on plan assets	(1)	(2)	(2)	(4)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	(23)		(46)
Defined Benefit Plan, Curtailments	0		0
Net periodic (benefit) cost	(22)	5	(45)	10
Other Postretirement Benefit Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1	2	1	4
Interest cost	0	4	1	9
Expected return on plan assets	(1)	(2)	(2)	(4)
Amortization of net actuarial loss	0	0	0	0
Amortization of prior service cost	(23)		(46)
Defined Benefit Plan, Curtailments	0		0
Net periodic (benefit) cost	(23)	4	(46)	9
Other Postretirement Benefit Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	1	1	1	1
Interest cost	0	0	0	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
Defined Benefit Plan, Curtailments	0		0
Net periodic (benefit) cost	$ 1	$ 1	$ 1	$ 1