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Summary of Treasury Stock Activity (Parenthetical) (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule of Treasury Stock [Line Items]      
Common stock repurchased value as result of restricted stock forfeitures $ 0.001 $ 0.001 $ 0.001
Minimum      
Schedule of Treasury Stock [Line Items]      
Market Value of Restricted Shares 29.17 29.17 29.17
Maximum      
Schedule of Treasury Stock [Line Items]      
Market Value of Restricted Shares $ 44.44 $ 44.44 $ 44.44