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Share Based Compensation (Additional Information) (Detail) - USD ($)
shares in Millions, $ in Millions
6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Stock Option    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock options granted (in shares) 0.4 0.5
Total unrecognized compensation cost $ 13.5  
Weighted-average period of total unrecognized compensation cost, in years 1 year 6 months  
Unvested Shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted (in shares) 0.1 0.2
Total unrecognized compensation cost $ 11.7  
Weighted-average period of total unrecognized compensation cost, in years 1 year 1 month 6 days  
Performance Shares    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted (in shares) 0.1 0.1
Total unrecognized compensation cost $ 8.7  
Weighted-average period of total unrecognized compensation cost, in years 1 year 1 month 6 days