Postretirement benefits Schedule of net periodic benefit costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Total (increase) decrease in other comprehensive (Income) Loss	$ 321	$ 16	$ (545)
Pension Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	69	80	93
Defined Benefit Plan, Non Service (Benefit) Cost	(124)	(123)	(188)
Net Periodic Benefit Cost	(55)	(43)	(95)
Pension Plan [Member] | UNITED STATES
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	57	63	75
Interest Cost	105	105	103
Expected Return on Plan Assets	(212)	(189)	(165)
Amortization of (Gains) Losses	0	0	0
Amortization of Prior Service Cost (Credit)	0	0	0
Recognized Net (Gain) Loss Due to Curtailments	0	0	(117)
Recognized Net (Gain) Loss Due to Settlements	2	0	(2)
Defined Benefit Plan, Non Service (Benefit) Cost	(105)	(84)	(181)
Net Periodic Benefit Cost	(48)	(21)	(106)
(Increase) decrease in Other Comprehensive Income (Loss), Net Actuarial (Gain) Loss)	214	(21)	(326)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	0	0
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	0	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	0	0	0
(Increase) decrease in other comprehensive income, curtailment	0	0	117
(Increase) decrease in other comprehensive income, settlement	(2)	1	2
Total (increase) decrease in other comprehensive (Income) Loss	212	(20)	(207)
Pension Plan [Member] | Non-US [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	12	17	18
Interest Cost	27	27	30
Expected Return on Plan Assets	(50)	(42)	(39)
Amortization of (Gains) Losses	1	3	2
Amortization of Prior Service Cost (Credit)	0	0	(1)
Recognized Net (Gain) Loss Due to Curtailments	0	(27)	0
Recognized Net (Gain) Loss Due to Settlements	3	0	1
Defined Benefit Plan, Non Service (Benefit) Cost	(19)	(39)	(7)
Net Periodic Benefit Cost	(7)	(22)	11
(Increase) decrease in Other Comprehensive Income (Loss), Net Actuarial (Gain) Loss)	34	(57)	49
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	3	0	(8)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	(1)	(3)	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	0	0	0
(Increase) decrease in other comprehensive income, curtailment	0	(6)	0
(Increase) decrease in other comprehensive income, settlement	(3)	0	(1)
Total (increase) decrease in other comprehensive (Income) Loss	33	(66)	40
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	1	2	10
Interest Cost	3	4	17
Expected Return on Plan Assets	(5)	(5)	(8)
Amortization of (Gains) Losses	0	0	(1)
Amortization of Prior Service Cost (Credit)	(85)	(89)	(15)
Recognized Net (Gain) Loss Due to Curtailments	(2)	(37)	0
Recognized Net (Gain) Loss Due to Settlements	0	0	0
Defined Benefit Plan, Non Service (Benefit) Cost	(89)	(127)	(7)
Net Periodic Benefit Cost	(88)	(125)	3
(Increase) decrease in Other Comprehensive Income (Loss), Net Actuarial (Gain) Loss)	(11)	(3)	17
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), before Tax	0	(23)	(395)
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss), Reclassification Adjustment from AOCI, before Tax	(1)	0	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, before Tax	85	89	0
(Increase) decrease in other comprehensive income, curtailment	3	39	0
(Increase) decrease in other comprehensive income, settlement	0	0	0
Total (increase) decrease in other comprehensive (Income) Loss	76	102	(378)
Losses and loss expenses [Member] | Pension Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	7	7	8
Defined Benefit Plan, Non Service (Benefit) Cost	(10)	(8)	(18)
Losses and loss expenses [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	0	0	0
Defined Benefit Plan, Non Service (Benefit) Cost	(9)	(13)	0
General and Administrative Expense [Member] | Pension Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	62	73	85
Defined Benefit Plan, Non Service (Benefit) Cost	(114)	(115)	(170)
General and Administrative Expense [Member] | Other Postretirement Benefits Plan [Member]
Defined Benefit Plan, Amount Recognized in Net Periodic Benefit Cost (Credit) and Other Comprehensive (Income) Loss, before Tax [Abstract]
Service Cost	1	2	10
Defined Benefit Plan, Non Service (Benefit) Cost	$ (80)	$ (114)	$ (7)