Stock Option Activity (Detail) - $ / shares	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Outstanding Options at Beginning of Year	1,323,379	1,817,763	1,665,565
Number of Shares, Granted	119,986	202,364	318,572
Number of Shares, Exercised	(448,295)	(677,566)	(137,874)
Number of Shares, Cancelled	(36,934)	(19,182)	(28,500)
Number of Shares, Outstanding Options at End of Year	958,136	1,323,379	1,817,763
Number of Shares, Options Exercisable at End of Year	651,218	874,116	1,285,412
Weighted-Average Exercise Price, Outstanding Options at Beginning of Year	$ 66.07	$ 53.03	$ 46.37
Weighted-Average Exercise Price, Granted	100.20	77.76	79.83
Weighted-Average Exercise Price, Exercised	60.27	34.46	27.58
Weighted-Average Exercise Price, Cancelled	79.97	70.30	86.35
Weighted-Average Exercise Price, Outstanding Options at End of Year	72.52	66.07	53.03
Weighted Average Fair Value of Options Granted during the Year	$ 33.37	$ 25.22	$ 24.27