Long-Term Debt (Senior Notes) (Details) - Senior notes $ in Millions  | 
 Mar. 31, 2018  
USD ($) 
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| Debt Instrument [Line Items] | |
| Aggregate unamortized premium | $ 31.7 | 
| 2021 Senior Notes | |
| Debt Instrument [Line Items] | |
| Interest rate | 6.75% | 
| 2025 Senior Notes | |
| Debt Instrument [Line Items] | |
| Interest rate | 6.875% | 
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. No definition available. 
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- Definition Amount, after accumulated amortization, of debt premium. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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