Stock-Based Employee Benefit Plans - Summary of Option Activity (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Shares
Outstanding at October 1, 2017 (in shares) | shares	1,230
Granted (in shares) | shares	29
Exercised (in shares) | shares	(257)
Forfeited (in shares) | shares	(6)
Outstanding at September 30, 2018 (in shares) | shares	996
Options exercisable at September 30, 2018 (in shares) | shares	847
Vested and expected to vest at September 30, 2018 (in shares) | shares	993
Weighted- average Exercise Price
Outstanding at October 1, 2017 (in dollars per share) | $ / shares	$ 56.54
Granted (in dollars per share) | $ / shares	167.27
Exercised (in dollars per share) | $ / shares	42.85
Forfeited (in dollars per share) | $ / shares	88.17
Outstanding at September 30, 2018 (in dollars per share) | $ / shares	63.13
Options exercisable at September 30, 2018 (in dollars per share) | $ / shares	59.10
Vested and expected to vest at September 30, 2018 (in dollars per share) | $ / shares	$ 62.97
Weighted- average Remaining Contractual Term
Outstanding at September 30, 2018	2 years 6 months 2 days
Exercisable at September 30, 2018	2 years 3 months 23 days
Vested and expected to vest at September 30, 2018	2 years 6 months
Aggregate Intrinsic Value
Outstanding at September 30, 2018 | $	$ 164,716
Exercisable at September 30, 2018 | $	143,529
Vested and expected to vest at September 30, 2017 | $	$ 164,500