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Share-based Payments - Movements in Number of Restricted Share Units Dependent on Market Performance Condition Outstanding under 2006 and 2016 Incentive Award Plan and Related Weighted Average Fair Values (Detail) - RSUs dependent on market performance condition [member]
12 Months Ended
Mar. 31, 2019
USD ($)
yr
Mar. 31, 2018
USD ($)
yr
Mar. 31, 2017
USD ($)
yr
Disclosure of terms and conditions of share-based payment arrangement [Line Items]      
Beginning balance 203,990 203,990  
Granted 0 0  
Exercised 0 0  
Forfeited 0 0  
Lapsed 0 0  
Ending balance 203,990 203,990 203,990
Weighted average fair value, Beginning balance $ 13.21 $ 13.21  
Exercisable 5,000    
Weighted average fair value, Granted $ 0 0  
Weighted average fair value, Ending balance 13.21 $ 13.21 $ 13.21
Exercisable $ 14.30    
Weighted average remaining contract term (in years) | yr 6.10 7.10 8.10
Weighted average remaining contract term (in years), exercisable | yr 7.08    
Aggregate intrinsic value $ 10,867,000 $ 9,247,000 $ 5,836,000
Aggregate intrinsic value, exercisable $ 266,000