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Performance Contingent TSR Shares (Detail) (Total Shareholders Return, USD $)
4 Months Ended
Apr. 20, 2013
Grant Date First January Twenty Thirteen
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares granted 276,100
Vesting date Mar. 01, 2015
Fair value per share $ 25.83
Grant Date Sixteen July Two Thousand And Twelve
 
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares granted 137,500
Vesting date Feb. 28, 2014
Fair value per share $ 23.18