Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 15	$ 18	$ 30	$ 35
Non-service (benefit) cost	(22)	(31)	(43)	(64)
Net periodic (benefit) cost	(7)	(13)	(13)	(29)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	2	3	5	6
Interest cost	7	7	14	14
Expected return on plan assets	(11)	(13)	(22)	(26)
Defined Benefit Plan, Amortization of Loss	1	0	1	0
Amortization of prior service cost	0	0	0	0
Curtailments	0	0	0	0
Settlements	0	0	0	0
Non-service (benefit) cost	(3)	(6)	(7)	(12)
Net periodic (benefit) cost	(1)	(3)	(2)	(6)
Pension Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	13	15	25	29
Interest cost	29	27	59	53
Expected return on plan assets	(48)	(53)	(95)	(106)
Defined Benefit Plan, Amortization of Loss	0	0	0	0
Amortization of prior service cost	0	0	0	0
Curtailments	0	0	0	0
Settlements	0	1	0	1
Non-service (benefit) cost	(19)	(25)	(36)	(52)
Net periodic (benefit) cost	(6)	(10)	(11)	(23)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Interest cost	1	1	2	2
Expected return on plan assets	(1)	(1)	(2)	(2)
Defined Benefit Plan, Amortization of Loss	0	0	0	0
Amortization of prior service cost	(20)	(22)	(40)	(43)
Curtailments	0	(1)	0	(1)
Settlements	0	0	0	0
Non-service (benefit) cost	(20)	(23)	(40)	(44)
Net periodic (benefit) cost	(20)	(23)	(40)	(44)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	1	3	3
Non-service (benefit) cost	(2)	(2)	(4)	(5)
Losses and loss expenses | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Non-service (benefit) cost	(2)	(3)	(4)	(4)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	14	17	27	32
Non-service (benefit) cost	(20)	(29)	(39)	(59)
Administrative Expense | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Non-service (benefit) cost	$ (18)	$ (20)	$ (36)	$ (40)