Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 1	$ 15	$ 3	$ 45
Non-service (benefit) cost	(35)	(20)	(104)	(63)
Net periodic (benefit) cost	(34)	(5)	(101)	(18)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	0	0	0
Interest cost	1	1	2	3
Expected return on plan assets	(1)	(1)	(3)	(3)
Defined Benefit Plan, Amortization of Gain (Loss)	0	0	0	0
Amortization of prior service cost	(20)	(20)	(60)	(60)
Settlements	0	0	0	0
Non-service (benefit) cost	(20)	(20)	(61)	(60)
Net periodic (benefit) cost	(20)	(20)	(61)	(60)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	2	0	5
Non-service (benefit) cost	(3)	(1)	(9)	(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)	(2)	(6)	(6)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	13	3	40
Non-service (benefit) cost	(32)	(19)	(95)	(58)
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)	(18)	(55)	(54)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	1	3	3	8
Interest cost	6	6	17	20
Expected return on plan assets	(11)	(11)	(32)	(33)
Defined Benefit Plan, Amortization of Gain (Loss)	0	1	1	2
Amortization of prior service cost	1	0	1	0
Settlements	0	0	0	0
Non-service (benefit) cost	(4)	(4)	(13)	(11)
Net periodic (benefit) cost	(3)	(1)	(10)	(3)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	12	0	37
Interest cost	24	30	74	89
Expected return on plan assets	(56)	(47)	(168)	(142)
Defined Benefit Plan, Amortization of Gain (Loss)	0	0	0	0
Amortization of prior service cost	0	0	0	0
Settlements	1	1	3	1
Non-service (benefit) cost	(31)	(16)	(91)	(52)
Net periodic (benefit) cost	$ (31)	$ (4)	$ (91)	$ (15)