Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 1	$ 15
Non-service (benefit) cost	(34)	(21)
Net periodic (benefit) cost	(33)	(6)
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)	(20)
Settlements	0	0
Non-service (benefit) cost	(20)	(20)
Net periodic (benefit) cost	(20)	(20)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	2
Non-service (benefit) cost	(3)	(2)
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)	(2)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	13
Non-service (benefit) cost	(31)	(19)
Administrative Expense | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0
Non-service (benefit) cost	(18)	(18)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	3
Interest cost	6	7
Expected return on plan assets	(10)	(11)
Amortization of prior service cost	0	0
Settlements	0	0
Non-service (benefit) cost	(4)	(4)
Net periodic (benefit) cost	(3)	(1)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	12
Interest cost	25	30
Expected return on plan assets	(56)	(47)
Amortization of prior service cost	0	0
Settlements	1	0
Non-service (benefit) cost	(30)	(17)
Net periodic (benefit) cost	$ (30)	$ (5)