|**Leases (Tables)**                                                                    | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
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|**Leases [Abstract]**                                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Components of Lease Expense and Other Information                                      | The components of lease expense were as follows:Three Months Ended June 30,Six Months Ended June 30,(in millions)20192019Operating lease cost$7.0$14.0Sublease income(0.4)(0.8)Total lease cost$6.6$13.2Other information related to leases was as follows:Six Months Ended June 30,(in millions, except lease term and discount rate)2019Cash paid for amounts included in the measurement of lease liabilities:Operating cash flows from operating leases$15.4Right-of-use assets obtained in exchange for lease obligations:Operating leases$19.5Weighted average remaining lease term:Operating leases7.4 yearsWeighted average discount rate:Operating leases5.4%
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|                                                                                       | Three Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           
|(in millions)                                                                          | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Operating lease cost                                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Sublease income                                                                        | (0.4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total lease cost                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|                                                                                       | Six Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
|(in millions, except lease term and discount rate)                                     | 2019                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Cash paid for amounts included in the measurement of lease liabilities:                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating cash flows from operating leases                                             | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|Right-of-use assets obtained in exchange for lease obligations:                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating leases                                                                       | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
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|Weighted average remaining lease term:                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating leases                                                                       | 7.4 years                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|Weighted average discount rate:                                                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|Operating leases                                                                       | 5.4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   
|Future Minimum Lease Payments                                                          | Future minimum lease payments, net of sub-lease income, for all non-cancelable leases as ofJune 30, 2019were as follows:(in millions)2019$12.8202023.0202119.2202215.4202311.7202410.9Thereafter40.1Total minimum lease payments133.1Less imputed interest26.2Future minimum payments for leases, net of sublease rental income and imputed interest$106.9                                                                                                                                                                                                                                                                                                            
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|(in millions)                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
|2019                                                                                   | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|2020                                                                                   | 23.0                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2021                                                                                   | 19.2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2022                                                                                   | 15.4                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2023                                                                                   | 11.7                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|2024                                                                                   | 10.9                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Thereafter                                                                             | 40.1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Total minimum lease payments                                                           | 133.1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Less imputed interest                                                                  | 26.2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Future minimum payments for leases, net of sublease rental income and imputed interest | $                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     

X- DefinitionTabular disclosure of lessee's lease cost. Includes, but is not limited to, interest expense for finance lease, amortization of right-of-use asset for finance lease, operating lease cost, short-term lease cost, variable lease cost and sublease income.
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