Postretirement benefits Components of net periodic benefit costs (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	$ 15	$ 17	$ 45	$ 52
Non-service (benefit) cost	(20)	(32)	(63)	(96)
Net periodic (benefit) cost	(5)	(15)	(18)	(44)
Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	1	0	1
Interest cost	1	1	3	3
Expected return on plan assets	(1)	(2)	(3)	(4)
Defined Benefit Plan, Amortization of Loss	0	0	0	0
Amortization of prior service cost	(20)	(21)	(60)	(64)
Curtailments	0	(1)	0	(2)
Settlements	0	0	0	0
Non-service (benefit) cost	(20)	(23)	(60)	(67)
Net periodic (benefit) cost	(20)	(22)	(60)	(66)
Losses and loss expenses | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	2	2	5	5
Non-service (benefit) cost	(1)	(3)	(5)	(8)
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)	(6)	(7)
Administrative Expense | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	13	15	40	47
Non-service (benefit) cost	(19)	(29)	(58)	(88)
Administrative Expense | Other Postretirement Benefits Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	0	1	0	1
Non-service (benefit) cost	(18)	(20)	(54)	(60)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	3	3	8	9
Interest cost	6	6	20	20
Expected return on plan assets	(11)	(12)	(33)	(38)
Defined Benefit Plan, Amortization of Loss	1	1	2	1
Amortization of prior service cost	0	0	0	0
Curtailments	0	0	0	0
Settlements	0	0	0	0
Non-service (benefit) cost	(4)	(5)	(11)	(17)
Net periodic (benefit) cost	(1)	(2)	(3)	(8)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans
Service cost	12	14	37	43
Interest cost	30	26	89	79
Expected return on plan assets	(47)	(53)	(142)	(159)
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	1	0	1	1
Non-service (benefit) cost	(16)	(27)	(52)	(79)
Net periodic (benefit) cost	$ (4)	$ (13)	$ (15)	$ (36)