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Equity (Tables) | 12 Months Ended
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Dec. 31, 2017
Equity [Abstract] |
Summary of common stock | The following table describes the number of shares authorized, issued and outstanding of our common stock as of December 31, 2017 and 2016:
| As of December 31, 2017 |  | As of December 31, 2016
(In millions) | Authorized |  | Issued |  | Outstanding |  | Authorized |  | Issued |  | Outstanding
Common stock | 1,000.0 |  |  | 235.3 |  |  | 211.5 |  |  | 1,000.0 |  |  | 238.5 |  |  | 215.9 |

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