Stock-Based Employee Benefit Plans - Summary of Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares
Shares
Outstanding at September 30, 2019 (in shares) | shares	616
Granted (in shares) | shares	32
Exercised (in shares) | shares	(401)
Forfeited (in shares) | shares	(1)
Outstanding at September 30, 2020 (in shares) | shares	246
Options exercisable at September 30, 2020 (in shares) | shares	168
Vested and expected to vest at September 30, 2020 (in shares) | shares	243
Weighted- average Exercise Price
Outstanding at September 30, 2019 (in dollars per share) | $ / shares	$ 89.36
Granted (in dollars per share) | $ / shares	364.23
Exercised (in dollars per share) | $ / shares	63.47
Forfeited (in dollars per share) | $ / shares	185.05
Outstanding at September 30, 2020 (in dollars per share) | $ / shares	166.80
Options exercisable at September 30, 2020 (in dollars per share) | $ / shares	130.87
Vested and expected to vest at September 30, 2020 (in dollars per share) | $ / shares	$ 165.32
Weighted- average Remaining Contractual Term
Outstanding at September 30, 2020	3 years 10 months 2 days
Exercisable at September 30, 2020	3 years 21 days
Vested and expected to vest at September 30, 2018	3 years 9 months 25 days
Aggregate Intrinsic Value
Outstanding at September 30, 2020 | $	$ 63,605
Exercisable at September 30, 2020 | $	49,435
Vested and expected to vest at September 30, 2017 | $	$ 63,125