Stock Option Activity (Detail)	6 Months Ended
	Sep. 30, 2018 $ / shares shares
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares, Outstanding Options at Beginning of Year | shares	958,136
Number of Shares, Granted | shares	115,734
Number of Shares, Exercised | shares	(35,454)
Number of Shares, Cancelled | shares	(2,197)
Number of Shares, Outstanding Options at End of Year | shares	1,036,219
Number of Shares, Options Exercisable at End of Year | shares	698,207
Weighted-Average Exercise Price, Outstanding Options at Beginning of Year	$ 72.52
Weighted-Average Exercise Price, Granted	106.14
Weighted-Average Exercise Price, Exercised	110.89
Weighted-Average Exercise Price, Cancelled	100.88
Weighted-Average Exercise Price, Outstanding Options at End of Year	76.77
Weighted-Average Exercise Price, Options Exercisable at End of Period	69.16
Weighted Average Fair Value of Options Granted during the Year	$ 33.99