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Debt Schedule of Commercial Paper (Details) - Commercial paper - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |
Commercial paper outstanding | $ 399 |  | $ 0
Weighted average commercial paper outstanding | $ 125 | $ 367 |
Weighted average interest rate of commercial paper outstanding | (0.50%) | 0.12% |

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