Stock Options and Stock-Based Compensation - Outstanding Options (Detail) - USD ($)	12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options outstanding, beginning balance, Shares	1,064,439	1,143,928	1,115,465
Granted, Number of Options	290,300	334,200	267,950
Exercised, Number of Options	(250,604)	(314,846)	(115,592)
Cancelled - forfeited, Number of Options	(44,508)
Cancelled - expired, Number of Options	(1,216)
Options outstanding, ending balance, Shares	1,058,411	1,064,439	1,143,928
Options vested and expected to vest, Number of Options	1,002,167
Ending exercisable, Number of Options	440,386
Options outstanding, beginning balance, Weighted Average Exercise Price	$ 39.45	$ 32.02
Granted, Weighted Average Exercise Price per Share	57.27	52.57	$ 37.13
Exercised, Weighted Average Exercise Price per Share	36.44	27.59	22.74
Cancelled - forfeited, Weighted Average Exercise Price per Share	36.67
Cancelled - expired, Weighted Average Exercise Price per Share	38.07
Options outstanding, ending balance, Weighted Average Exercise Price	45.17	$ 39.45	$ 32.02
Options vested and expected to vest, Weighted Average Exercise Price per Share	44.42
Ending exercisable, Weighted Average Exercise Price per Share	$ 35.73
Option outstanding, Weighted Average Remaining Contractual Life (Years)	3 years 5 months 19 days
Options vested and expected to vest, Weighted Average Remaining Contractual Life (Years)	3 years 4 months 28 days
Ending exercisable, Weighted Average Remaining Contractual Life (Years)	2 years 8 months 8 days
Option outstanding, Aggregate Intrinsic Value	$ 21,239,346
Options vested and expected to vest, Aggregate Intrinsic Value	20,004,421
Ending exercisable, Aggregate Intrinsic Value	$ 12,994,093