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Condensed Consolidating Financial Information (Narrative) (Details) - USD ($) | Mar. 31, 2018 | Oct. 11, 2017 | Dec. 21, 2016 | Oct. 14, 2016 | Apr. 21, 2015
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6.625% Senior Notes due 2023 | Senior notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, amount |  |  |  |  | $ 350,000,000.0
6.000% Senior Notes due 2024 | Senior notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, amount |  |  |  | $ 650,000,000.0 |
6.375% Senior Notes due 2025 | Senior notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, amount |  |  | $ 600,000,000.0 |  |
6.375% Senior Notes Due 2026 | Senior notes |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Debt instrument, amount |  | $ 450,000,000.0 |  |  |
Guarantors |  |  |  |  |
Condensed Financial Statements, Captions [Line Items] |  |  |  |  |
Ownership percentage by parent | 100.00% |  |  |  |

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