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Share Based Compensation - Performance Share Units - Narrative (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2014
Mar. 31, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2012
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Share-based compensation $ 3.9   $ 4.5 $ 13.8 $ 15.6  
Performance Shares
           
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]            
Performance share unit awards granted to certain executive officers       319,000    
Equity instruments other than options, payout range, minimum       154,500    
Equity instruments other than options, payout range, maximum       319,000 485,000 233,000
Compensation expense achieved target, percentage   52.00%        
Compensation expense achieved target, shares   121,431        
Share-based compensation 1.0   0.6 2.6 1.7  
Unrecognized share-based compensation cost $ 6.2     $ 6.2    
Unrecognized compensation expense, weighted-average period of recognition       1 year 10 months 24 days