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Capital Stock (Tables) | 12 Months Ended
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Dec. 31, 2017
Stockholders Equity Note [Abstract] |
Schedule of Common Stock Outstanding | Common stock outstanding as of December 31, 2017, 2016 and 2015 was as follows:
(Number of shares in millions) |  | 2017 |  |  | 2016 |  |  | 2015 |
Common stock: |  |  |  |  |  |  |  |  |  |  |  |
Balance, beginning of year |  | 106.7 |  |  |  | 106.0 |  |  | 104.8 |
Activity under stock plans |  | 1.1 |  |  | 0.7 |  |  | 1.2 |
Balance, end of year |  | 107.8 |  |  | 106.7 |  |  |  | 106.0 |
Treasury stock: |  |  |  |  |  |  |  |  |  |  |  |
Balance, beginning of year |  | 15.3 |  |  | 12.5 |  |  | 9.3 |
Repurchased |  |  | 2.9 |  |  | 2.8 |  |  | 3.2 |
Balance, end of year |  | 18.2 |  |  | 15.3 |  |  | 12.5 |
Common stock outstanding |  | 89.6 |  |  | 91.4 |  |  | 93.5 |

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