Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 15	$ 17
Non-service (benefit) cost	(21)	(33)
Net periodic (benefit) cost	(6)	(16)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	3	3
Interest cost	7	7
Expected return on plan assets	(11)	(13)
Amortization of prior service cost	0	0
Non-service (benefit) cost	(4)	(6)
Net periodic (benefit) cost	(1)	(3)
Pension Plan [Member] | UNITED STATES
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	12	14
Interest cost	30	26
Expected return on plan assets	(47)	(53)
Amortization of prior service cost	0	0
Non-service (benefit) cost	(17)	(27)
Net periodic (benefit) cost	(5)	(13)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0
Interest cost	1	1
Expected return on plan assets	(1)	(1)
Amortization of prior service cost	(20)	(21)
Non-service (benefit) cost	(20)	(21)
Net periodic (benefit) cost	(20)	(21)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	2	2
Non-service (benefit) cost	(2)	(3)
Losses and loss expenses | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0
Non-service (benefit) cost	(2)	(1)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	13	15
Non-service (benefit) cost	(19)	(30)
Administrative Expense | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0
Non-service (benefit) cost	$ (18)	$ (20)