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Stock-Based Compensation (Tables) | 12 Months Ended
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Dec. 31, 2017
Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
Restricted shares, performance-based restricted share units and performance share unit activity | The following table presents activity for non-vested RSUs, performance-based RSUs and performance share units under the Plan in 2017.
| Number of Awards |  | Weighted-Average Grant Date Fair Value
Balance as of January 1, 2017 | 2,131,782 |  |  | $ | 33.28 |
Awards granted | 1,010,741 |  |  | 44.20 |
Awards vested | (930,616 | ) |  | 33.31 |
Awards forfeited, canceled or expired | (290,296 | ) |  | 36.20 |
Performance-based adjustment | 149,126 |  |  | 44.07 |
Balance as of December 31, 2017 | 2,070,737 |  |  | $ | 38.92 |

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- Definition Disclosure of the number and weighted-average grant date fair value for restricted stock and restricted stock units that were outstanding at the beginning and end of the year, and the number of restricted stock and restricted stock units that were granted, vested, or forfeited during the year. + References No definition available. + Details
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