|**Segment Reporting - Table 4 (Details) - USD ($) $ in Millions**                                                                                                                                                                                                                                                                                                                                                                                                                           | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                          | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
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|**Segment Reporting, Asset Reconciling Item [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Total assets for reportable segments                                                                                                                                                                                                                                                                                                                                                                                                                                                        | $ 5,131.9                                                                                                                                                                                                                                                                                                                                                                                                                                                                               | [1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Reclassification of noncurrent deferred income tax liabilities to net noncurrent deferred income tax assets                                                                                                                                                                                                                                                                                                                                                                                 | (40.5)                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Total assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | 5,131.9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 | [1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    
|Operating Segments [Member]                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|**Segment Reporting, Asset Reconciling Item [Line Items]**                                                                                                                                                                                                                                                                                                                                                                                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Total assets for reportable segments                                                                                                                                                                                                                                                                                                                                                                                                                                                        | 5,172.4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|Total assets                                                                                                                                                                                                                                                                                                                                                                                                                                                                                | $ 5,172.4                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
|                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|[1]
Our consolidated assets as of September 30, 2018 and December 31, 2017 include total assets of variable interest entities of $288.6 million and $264.1 million, respectively, which cannot be used by us to settle the obligations of other entities. Our consolidated liabilities as of September 30, 2018 and December 31, 2017 include total liabilities of the variable interest entities of $81.5 million and $52.5 million, respectively. See Note 3, Variable Interest Entities. | [1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     | Our consolidated assets as of September 30, 2018 and December 31, 2017 include total assets of variable interest entities of $288.6 million and $264.1 million, respectively, which cannot be used by us to settle the obligations of other entities. Our consolidated liabilities as of September 30, 2018 and December 31, 2017 include total liabilities of the variable interest entities of $81.5 million and $52.5 million, respectively. See Note 3, Variable Interest Entities.
|[1]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         | Our consolidated assets as of September 30, 2018 and December 31, 2017 include total assets of variable interest entities of $288.6 million and $264.1 million, respectively, which cannot be used by us to settle the obligations of other entities. Our consolidated liabilities as of September 30, 2018 and December 31, 2017 include total liabilities of the variable interest entities of $81.5 million and $52.5 million, respectively. See Note 3, Variable Interest Entities.

X- DefinitionSum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.
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