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Summary of Activity for Nonvested Restricted Shares (Detail) - Restricted Stock [Member] - $ / shares
12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Number of Shares, Beginning of Year 371,213    
Number of Shares, Granted 107,964    
Number of Shares, Vested (119,187)    
Number of Shares, Forfeited (31,931)    
Number of Shares, Nonvested at End of Year 328,059 371,213  
Weighted Average Grant Date Fair Value, Beginning of Year $ 60.33    
Weighted Average Grant Date Fair Value, Granted 100.05 $ 76.67 $ 81.30
Weighted Average Grant Date Fair Value, Vested 80.04    
Weighted Average Grant Date Fair Value, Forfeited 40.00    
Weighted Average Grant Date Fair Value, Nonvested at End of Year $ 65.76 $ 60.33