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Postretirement benefits (Narrative) (Details) - USD ($) $ in Millions |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Oct. 31, 2016 | Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Expenses recognized during period under the defined contributions plans |  |  | $ 171 | $ 166 | $ 150
Available for sale, Fair Value |  |  | 78,470 | 78,939 |
Defined benefit plan, benefits paid inclusive of settlements |  |  | $ 209 | 200 |
Minimum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage |  |  | 55.00% |  |
Maximum |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Investment within Plan Asset Category, Percentage |  |  | 65.00% |  |
Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Accumulated Benefit Obligation |  |  | $ 4,000 | 4,300 |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  |  | 22 |  |
Other Postretirement Benefits Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | $ 159 | 143 | 157 | 159
Actuarial Loss (gain) |  |  | (20) | (2) |
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation |  |  | 23 | 21 |
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year |  |  | 1 |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
(Increase) decrease in other comprehensive income, curtailment |  |  | 3 | 39 | 0
Amortization of Prior Service Cost (Credit) |  |  | 85 | 89 | 15
Non-US [Member] | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 962 | 1,008 | 1,172 | 962
Actuarial Loss (gain) |  |  | (71) | (4) |
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation |  |  | 115 | 152 |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
(Increase) decrease in other comprehensive income, curtailment |  |  | 0 | (6) | 0
Amortization of Prior Service Cost (Credit) |  |  | 0 | 0 | 1
UNITED STATES |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation |  | 496 |  |  |
UNITED STATES | Pension Plan [Member] |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Defined Benefit Plan, Plan Assets, Amount |  | 2,765 | 2,784 | 3,109 | 2,765
Actuarial Loss (gain) |  |  | (214) | 232 |
Defined Benefit Plan, Plan with Accumulated Benefit Obligation in Excess of Plan Assets, Accumulated Benefit Obligation |  |  | 3,066 | 3,223 |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
(Increase) decrease in other comprehensive income, curtailment |  |  | 0 | 0 | 117
Effect of Plan Amendment on Accumulated Benefit Obligation |  | $ 113 |  |  |
Amortization of Prior Service Cost (Credit) |  |  | 0 | 0 | 0
UNITED STATES | Postretirement Health Coverage [Member] |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
(Increase) decrease in other comprehensive income, curtailment |  |  | 3 | 39 |
Effect of Plan Amendment on Accumulated Benefit Obligation | $ 383 |  |  |  |
Amortization of Prior Service Cost (Credit) |  |  | 80 | $ 89 | $ 15
Remaining Balance of Change in Benefit Obligation For Plan Amendment |  |  | $ 184 |  |
UNITED STATES | Postretirement Health Coverage [Member] | Subject to Amortization [Member] |  |  |  |  |
Defined Benefit Plan, Plan Amendment [Abstract] |  |  |  |  |
Effect of Plan Amendment on Accumulated Benefit Obligation | $ 410 |  |  |  |

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