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Debt - 2014 Senior Notes (Details) - 2014 Senior Notes - USD ($) | Oct. 06, 2014 | Dec. 31, 2017
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The 2022 Notes |  |
Debt |  |
Principal amount | $ 800,000,000 |
Interest rate (as a percent) | 5.75% |
Fair value of debt instrument |  | $ 834,000,000
The 2024 Notes |  |
Debt |  |
Principal amount | $ 800,000,000 |
Interest rate (as a percent) | 5.875% |
Fair value of debt instrument |  | $ 866,000,000
The 2024 Notes | Prior to July 15 2024 |  |
Debt |  |
Redemption price equal to principal amount (in percent) | 100.00% |
The 2024 Notes | On or after July 15, 2024 |  |
Debt |  |
Redemption price equal to principal amount (in percent) | 100.00% |

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