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Stock-Based Compensation (Detail) (USD $)
In Millions, unless otherwise specified
1 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended
Feb. 29, 2012
Performance-Based Restricted Stock Units Granted In 2012 [Member]
Sep. 30, 2012
Performance-Based Restricted Stock Units Granted In 2012 [Member]
Mar. 31, 2012
Performance-Based Restricted Stock Units Granted In 2011 [Member]
Sep. 30, 2012
Performance-Based Restricted Stock Units Granted In 2011 [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Performance-based restricted stock units awarded 0.9      
Percentage of performance awards vested upon achievement of performance goal   50.00%    
Percentage of restricted stock units vested one year following achievement of performance goal   50.00%    
Additional stock-based compensation that would have been recorded if operating income targets had been deemed probable   $ 7.2   $ 3.5
Compensation expense for performance based restricted stock units awarded, cumulative adjustment     $ 2.4 $ 7.0