Stock-Based Employee Benefit Plans - Summary of Option Activity (Details) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2019 USD ($) $ / shares shares
Shares
Outstanding at October 1, 2017 (in shares) | shares	996
Granted (in shares) | shares	81
Exercised (in shares) | shares	(456)
Forfeited (in shares) | shares	(5)
Outstanding at September 30, 2018 (in shares) | shares	616
Options exercisable at September 30, 2018 (in shares) | shares	533
Vested and expected to vest at September 30, 2018 (in shares) | shares	613
Weighted- average Exercise Price
Outstanding at October 1, 2017 (in dollars per share) | $ / shares	$ 63.13
Granted (in dollars per share) | $ / shares	196.43
Exercised (in dollars per share) | $ / shares	50.03
Forfeited (in dollars per share) | $ / shares	178.09
Outstanding at September 30, 2018 (in dollars per share) | $ / shares	89.36
Options exercisable at September 30, 2018 (in dollars per share) | $ / shares	73.89
Vested and expected to vest at September 30, 2018 (in dollars per share) | $ / shares	$ 88.84
Weighted- average Remaining Contractual Term
Outstanding at September 30, 2019	2 years 9 months 3 days
Exercisable at September 30, 2019	2 years 3 months
Vested and expected to vest at September 30, 2018	2 years 8 months 26 days
Aggregate Intrinsic Value
Outstanding at September 30, 2019 | $	$ 131,921
Exercisable at September 30, 2019 | $	122,381
Vested and expected to vest at September 30, 2017 | $	$ 131,509