| **Earnings Per Share**            | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
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| **Earnings Per Share [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Earnings Per Share                | 11. Earnings per ShareTotal outstanding shares as of June 30, 2016 and 2015 were 107.2 million and 108.1 million, respectively. Under our stock repurchase program, for the three and six months ended June 30, 2015 we repurchased and retired 3.0 million and 5.1 million shares, respectively, of common stock. No shares were repurchased during the three or six months ended June 30, 2016. In addition, for the three and six months ended June 30, 2016 and 2015 we acquired 0.0 million and 0.1 million shares and 0.7 million and 0.7 million shares, respectively, of treasury shares related to tax withholding for share-based compensation. The calculation of basic earnings per common share is based on the weighted average number of common shares outstanding in the year, or period, over which they were outstanding. Our calculation of diluted earnings per common share assumes that any common shares outstanding were increased by shares that would be issued upon exercise of those stock units for which the average market price for the period exceeds the exercise price less the shares that could have been purchased by the Company with the related proceeds, including compensation expense measured but not yet recognized, net of tax. Three Months Ended June 30, Six Months Ended June 30,(in millions)2016 2015 2016 2015Weighted-average number of common shares outstanding — basic107.1 109.1 106.6 110.6Stock options0.8 0.2 0.5 0.2Stock-settled stock appreciation rights— 0.3 — 0.4Restricted stock units1.1 1.0 1.5 1.3Adjusted weighted-average shares and assumed conversions — diluted109.0 110.6 108.6 112.5Awards of potentially dilutive shares of common stock, which have exercise prices that were higher than the average market price during the period, are not included in the computation of dilutive earnings per share as their effect would have been anti-dilutive. For the three and six months ended June 30, 2016 these shares were approximately 3.8 million and 6.4 million, respectively. For the three and six months ended June 30, 2015 these shares were approximately 6.0 million and 5.2 million, respectively. |

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| - Definition
The entire disclosure for earnings per share.
+ References
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