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Share Based Compensation - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock options granted 0.6 0.7
Shares granted 0.2 0.2
Period of payment of award 3 years  
Minimum
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Payment of award, percentage 0.00%  
Maximum
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Payment of award, percentage 200.00%  
Stock Option
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized compensation cost 12.4  
Weighted-average period of total unrecognized compensation cost, in years 1 year 7 months 6 days  
Unvested Shares
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total unrecognized compensation cost 15.6  
Weighted-average period of total unrecognized compensation cost, in years 1 year 3 months 18 days  
Performance Shares
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Shares granted 0.1  
Total unrecognized compensation cost 2.3  
Weighted-average period of total unrecognized compensation cost, in years 1 year 6 months