|**Stock-Based Compensation (Tables)**                                                           | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                       
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|**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 in2017.Number of AwardsWeighted-Average Grant Date Fair ValueBalance as of January 1, 20172,131,782$33.28Awards granted1,010,74144.20Awards vested(930,616)33.31Awards forfeited, canceled or expired(290,296)36.20Performance-based adjustment149,12644.07Balance as of December 31, 20172,070,737$38.92
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|                                                                                                | Number of Awards                                                                                                                                                                                                                                                                                                                                                                                                                      
|Balance as of January 1, 2017                                                                   | 2,131,782                                                                                                                                                                                                                                                                                                                                                                                                                             
|Awards granted                                                                                  | 1,010,741                                                                                                                                                                                                                                                                                                                                                                                                                             
|Awards vested                                                                                   | (930,616                                                                                                                                                                                                                                                                                                                                                                                                                              
|Awards forfeited, canceled or expired                                                           | (290,296                                                                                                                                                                                                                                                                                                                                                                                                                              
|Performance-based adjustment                                                                    | 149,126                                                                                                                                                                                                                                                                                                                                                                                                                               
|Balance as of December 31, 2017                                                                 | 2,070,737                                                                                                                                                                                                                                                                                                                                                                                                                             

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