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Stock Options and Stock-Based Compensation - Outstanding Options (Detail) - USD ($)
 12 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Options outstanding, beginning balance, Shares 1,143,928 1,115,465 1,163,179
Granted, Number of Options 334,200 267,950 276,275
Exercised, Number of Options (314,846) (115,592) (200,753)
Cancelled - forfeited, Number of Options (81,586)    
Cancelled - expired, Number of Options (17,257)    
Options outstanding, ending balance, Shares 1,064,439 1,143,928 1,115,465
Options vested and expected to vest, Number of Options 959,664    
Ending exercisable, Number of Options 445,387    
Options outstanding, beginning balance, Weighted Average Exercise Price $ 32.02 $ 30.36  
Granted, Weighted Average Exercise Price per Share 52.57 37.13 $ 35.51
Exercised, Weighted Average Exercise Price per Share 27.59 22.74 19.75
Cancelled - forfeited, Weighted Average Exercise Price per Share 36.22    
Cancelled - expired, Weighted Average Exercise Price per Share 27.57    
Options outstanding, ending balance, Weighted Average Exercise Price 39.45 $ 32.02 $ 30.36
Options vested and expected to vest, Weighted Average Exercise Price per Share 39.76    
Ending exercisable, Weighted Average Exercise Price per Share $ 31.78    
Option outstanding, Weighted Average Remaining Contractual Life (Years) 3 years 4 months 28 days    
Options vested and expected to vest, Weighted Average Remaining Contractual Life (Years) 3 years 5 months 4 days    
Ending exercisable, Weighted Average Remaining Contractual Life (Years) 2 years 5 months 12 days    
Option outstanding, Aggregate Intrinsic Value $ 12,986,338    
Options vested and expected to vest, Aggregate Intrinsic Value 10,249,240    
Ending exercisable, Aggregate Intrinsic Value $ 8,360,918