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Debt - Schedule of Commercial Paper (Details) - Commercial paper - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Debt Instrument [Line Items] |  |  |  |  |
Commercial paper outstanding | $ 798 |  | $ 798 |  | $ 0
Weighted average commercial paper outstanding | $ 744 | $ 318 | $ 437 | $ 342 |
Weighted average interest rate of commercial paper outstanding | 0.92% | 0.26% | 0.85% | 0.18% |

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