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Stock-Based Compensation - Summary of Stock Option Activity (Detail) (Restricted Stock Units and Performance Share Awards [Member])
9 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Restricted Stock Units and Performance Share Awards [Member]
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Beginning Balance 847,679 788,088
Granted 210,856 207,728
Forfeited (40,991) (29,825)
Shares delivered (113,230) (95,430)
Ending Balance 904,314 870,561