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Debt - Public Debt (Details) - 5.72% Notes, due 2019 - USD ($) | 3 Months Ended |
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Mar. 31, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |
Long-term debt, interest rate | 5.72% | 5.72%
Long-term debt | $ 500,000,000 |
Redemption amount | 518,000,000 |
Loss on prepayment of debt redemption | $ 19,000,000 |

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