Stock-Based Awards - Stock Option Activity (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Options
Outstanding as of beginning of period	2,624
Granted	653
Exercised	(231)
Forfeited	(79)
Outstanding as of end of period	2,967	2,624
Options vested and expected to vest as of end of period	2,967
Options exercisable as of end of period	1,954
Weighted Average Exercise Price
Outstanding	$ 36.65
Granted	82.41
Exercised	29.68
Forfeited	63.68
Outstanding	46.54	$ 36.65
Options vested and expected to vest as of end of period	46.54
Options exercisable as of end of period	$ 37.09
Weighted Average Remaining Contractual Term
Outstanding as of end of period	6 years 8 months 12 days	7 years 1 month 6 days
Options vested and expected to vest as of end period	6 years 8 months 12 days
Options exercisable as of end of period	5 years 8 months 12 days
Aggregate Intrinsic Value
Aggregate Intrinsic Value	$ 57,336	$ 129,115
Options vested and expected to vest as of end of period	57,336
Options exercisable as of end of period	48,839
Aggregate intrinsic value of stock options exercised	13,855	14,180	$ 1,503
Proceeds to Company from stock options exercised	$ 6,848	$ 6,243	$ 1,079