Long-Term Debt (Fair Value of Debt) (Details) $ in Millions  | 
 Sep. 30, 2018  
USD ($) 
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| Senior Notes | |
| Debt Instrument [Line Items] | |
| Debt, fair value | $ 792.6 | 
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- Definition Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable. Reference 1: http://www.xbrl.org/2003/role/disclosureRef 
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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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- Details 
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