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Retirement Benefits (Reconciliation of Net Actuarial Loss) (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Pension Plan |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Total recognized as component of net periodic benefit credit |  | $ (130) | $ (62) | $ 82
Postretirement Benefits |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Total recognized as component of net periodic benefit credit | $ (125) | 6 | 9 | (118)
Domestic Plan | Pension Plan |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Effect of curtailment |  | 0 | (98) |
Total recognized as component of net periodic benefit credit |  | (56) | 65 | 141
Other |  | 0 | 0 |
Domestic Plan | Postretirement Benefits |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Effect of curtailment |  | 0 | 0 |
Total recognized as component of net periodic benefit credit |  | 4 | 4 | (124)
Other |  | 0 | (1) |
Domestic Plan | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent | Pension Plan |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Beginning balance |  | (1,720) | (1,754) |
Amortization of net loss |  | 37 | 74 |
Effect of curtailment |  | 0 | (98) |
Other |  | 0 | 0 |
Liability experience |  | (538) | (160) |
Asset experience |  | 455 | 22 |
Total amount recognized as change in plan assets and benefit obligations |  | (83) | (40) |
Ending balance, December 31 |  | (1,766) | (1,720) | (1,754)
Domestic Plan | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent | Postretirement Benefits |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Beginning balance |  | 11 | 13 |
Amortization of net loss |  | (1) | (2) |
Effect of curtailment |  | 0 | 0 |
Other |  | 1 | 0 |
Liability experience |  | (3) | 0 |
Asset experience |  | 0 | 0 |
Total amount recognized as change in plan assets and benefit obligations |  | (4) | 0 |
Ending balance, December 31 |  | 6 | 11 | 13
Foreign Plan | Pension Plan |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Effect of curtailment |  | (1) | (7) |
Total recognized as component of net periodic benefit credit |  | (74) | (127) | (59)
Other |  | 13 | 0 |
Foreign Plan | Postretirement Benefits |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Effect of curtailment |  | 0 | 0 |
Total recognized as component of net periodic benefit credit |  | 2 | 5 | 6
Other |  | 0 | 0 |
Foreign Plan | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent | Pension Plan |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Beginning balance |  | (3,081) | (2,887) |
Amortization of net loss |  | 130 | 94 |
Effect of settlement |  | 54 | 0 |
Total recognized as component of net periodic benefit credit |  | 184 | 94 |
Liability experience |  | 149 | (1,966) |
Asset experience |  | 311 | 1,254 |
Plan amendments |  | (5) | 0 |
Effect of curtailment |  | 1 | 3 |
Total amount recognized as change in plan assets and benefit obligations |  | 456 | (709) |
Exchange rate adjustments |  | (205) | 421 |
Ending balance, December 31 |  | (2,646) | (3,081) | (2,887)
Foreign Plan | Accumulated Defined Benefit Plans Adjustment, Net Gain (Loss) Attributable to Parent | Postretirement Benefits |  |  |  |
Reconciliation of unrecognized net actuarial gain loss [Roll Forward] |  |  |  |
Beginning balance |  | (11) | (6) |
Amortization of net loss |  | 1 | 0 |
Effect of settlement |  | 0 | 0 |
Total recognized as component of net periodic benefit credit |  | 1 | 0 |
Liability experience |  | 0 | (5) |
Asset experience |  | 0 | 0 |
Plan amendments |  | 0 | 0 |
Effect of curtailment |  | 0 | 0 |
Total amount recognized as change in plan assets and benefit obligations |  | 0 | (5) |
Exchange rate adjustments |  | 0 | 0 |
Ending balance, December 31 |  | $ (10) | $ (11) | $ (6)

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