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Share-based Payments - Movements in Number of Performance Share Units Outstanding under 2006 and 2016 Incentive Award Plan and Related Weighted Average Fair Values (Detail) - PSUs [member]
12 Months Ended
Mar. 31, 2018
USD ($)
yr
Mar. 31, 2017
USD ($)
yr
Mar. 31, 2016
USD ($)
yr
Disclosure of terms and conditions of share-based payment arrangement [Line Items]      
Beginning balance 1,184,852 951,588  
Granted 400,463 422,062  
Exercised (506,660) (141,741)  
Forfeited (18,681) (47,057)  
Ending balance 1,059,975 1,184,852 951,588
Weighted average fair value, Beginning balance $ 21.00 $ 17.83  
Exercisable 194,629    
Weighted average fair value, Granted $ 30.35 31.12  
Weighted average fair value, Exercised 16.79 14.11  
Weighted average fair value, Forfeited 29.38 26.27  
Weighted average fair value, Ending balance 26.36 $ 21.00 $ 17.83
Weighted average fair value, exercisable $ 18.14    
Weighted average remaining contract term (in years) | yr 7.51 7.36 7.77
Weighted average remaining contract term (in years), exercisable | yr 5.86    
Aggregate intrinsic value $ 48,048,000 $ 33,899,000 $ 29,157,000
Aggregate intrinsic value, exercisable $ 8,823,000