Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Mar. 31, 2017
Options
Outstanding at beginning of period	1,646
Granted	152
Exercised	(371)
Cancelled and expired	(55)
Outstanding at end of period	1,372	1,646
Exercisable at end of period	1,048
Options vested and expected to vest at end of period	1,347
Weighted-Average Exercise Price
Outstanding at beginning of period	$ 32.09
Granted	140.41
Exercised	20.54
Cancelled and expired	100.57
Outstanding at end of period	44.49	$ 32.09
Exercisable at end of period	24.03
Options vested and expected to vest at end of period	$ 43.85
Weighted-Average Remaining Contractual Term (years)
Outstanding	5 years 5 months 1 day	5 years 5 months 15 days
Exercisable at end of period	4 years 5 months 1 day
Options vested and expected to vest at end of period	5 years 4 months 13 days
Aggregate Intrinsic Value
Outstanding at end of period	$ 196,132
Exercisable at end of period	171,235
Options vested and expected to vest at end of period	$ 193,386