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Share-based Payments - Movements in Number of Restricted Share Units Dependent on Non-market Performance Condition Outstanding under 2006 and 2016 Incentive Award Plan and Related Weighted Average Fair Values (Detail) - RSUs dependent on non-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 1,072,833 1,573,246  
Granted 383,229 497,689  
Exercised (422,088) (961,111)  
Forfeited (70,723) (34,775)  
Lapsed (10,856) (2,216)  
Ending balance 952,395 1,072,833 1,573,246
Weighted average fair value, Beginning balance $ 27.05 $ 22.47  
Exercisable 410,948    
Weighted average fair value, Granted $ 47.89 30.85  
Weighted average fair value, Exercised 28.40 21.45  
Weighted average fair value, Forfeited 36.08 29.07  
Weighted average fair value, Lapsed 8.22 23.47  
Weighted average fair value, Ending balance 34.38 $ 27.05 $ 22.47
Weighted average fair value, exercisable $ 27.40    
Weighted average remaining contract term (in years) | yr 7.62 7.75 7.50
Weighted average remaining contract term (in years), exercisable | yr 6.33    
Aggregate intrinsic value $ 50,734,000 $ 48,632,000 $ 45,011,000
Aggregate intrinsic value, exercisable $ 21,891,000