|**Earnings Per Share (Tables)**                                       | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                               
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|**Earnings Per Share [Abstract]**                                     |                                                                                                                                                                                                                                                                                                                                                                                                                              
|Schedule of Weighted Average Number of Shares                         | Our weighted-average shares outstanding for thethree and sixmonths endedJune 30, 2019and2018was as follows:Three Months Ended June 30,Six Months Ended June 30,(in millions)2019201820192018Weighted-average number of shares of common stock outstanding - basic101.3106.1101.8106.4Stock options0.31.10.41.2Restricted stock units0.60.81.11.4Weighted-average shares and assumed conversions - diluted102.2108.0103.3109.0
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|                                                                      | Three Months Ended June 30,                                                                                                                                                                                                                                                                                                                                                                                                  
|(in millions)                                                         | 2019                                                                                                                                                                                                                                                                                                                                                                                                                         
|Weighted-average number of shares of common stock outstanding - basic | 101.3                                                                                                                                                                                                                                                                                                                                                                                                                        
|Stock options                                                         | 0.3                                                                                                                                                                                                                                                                                                                                                                                                                          
|Restricted stock units                                                | 0.6                                                                                                                                                                                                                                                                                                                                                                                                                          
|Weighted-average shares and assumed conversions - diluted             | 102.2                                                                                                                                                                                                                                                                                                                                                                                                                        

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