Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Sep. 30, 2017	Mar. 31, 2017
Options
Outstanding at beginning of period	1,646
Granted	145
Exercised	(305)
Cancelled and expired	(51)
Outstanding at end of period	1,435	1,646
Exercisable at end of period	1,098
Options vested and expected to vest at end of period	1,405
Weighted-Average Exercise Price
Outstanding at beginning of period	$ 32.09
Granted	137.70
Exercised	17.25
Cancelled and expired	100.48
Outstanding at end of period	43.47	$ 32.09
Exercisable at end of period	24.23
Options vested and expected to vest at end of period	$ 42.84
Weighted-Average Remaining Contractual Term (years)
Outstanding	5 years 6 months 22 days	5 years 5 months 16 days
Exercisable at end of period	4 years 6 months 25 days
Options vested and expected to vest at end of period	5 years 6 months 4 days
Aggregate Intrinsic Value
Outstanding at end of period	$ 179,543
Exercisable at end of period	158,474
Options vested and expected to vest at end of period	$ 176,658