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Retirement Benefits Retirement Benefits - Balance Sheet locations of Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Pension Plan, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Other current liabilities | $ 26 | $ 28
Pension obligations | 3,988 | 3,736
Non-pension postretirement benefit obligations | 0 | 0
Unfunded status (net benefit obligation less plan assets) | 4,014 | 3,764
Other Postretirement Benefit Plan, Defined Benefit [Member] |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Other current liabilities | 33 | 31
Pension obligations | 0 | 0
Non-pension postretirement benefit obligations | 1,928 | 2,093
Unfunded status (net benefit obligation less plan assets) | $ 1,961 | $ 2,124

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