Stock-Based Compensation - Summary of Stock Option Activity (Details) - Employee Stock Option - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Weighted- Average Exercise Price Per Share
Beginning of year (in dollars per share)	$ 58.10
Granted (in dollars per share)	0	$ 0	$ 0
Exercised (in dollars per share)	60.66
Canceled/Forfeited (in dollars per share)	53.05
End of year (in dollars per share)	53.35	$ 58.10
Exercisable at end of year(in dollars per share)	$ 53.35
Shares
Beginning of year (in shares)	1,692,000
Exercised (in shares)	(1,099,000)
Canceled/Forfeited (in shares)	(4,000)
End of year (in shares)	589,000	1,692,000
Exercisable at end of year (in shares)	589,000
Total Intrinsic Value of Shares
Total intrinsic value of awards, at end of year	$ 26
Total intrinsic value of awards, exercisable at end of year	$ 26
Average Remaining Life (Years)
Average remaining life, at end of year	2 years 1 month 6 days
Average remaining life, exercisable at year end	2 years 1 month 6 days
AspenTech
Weighted- Average Exercise Price Per Share
Beginning of year (in dollars per share)	$ 131.26
Granted (in dollars per share)	196.04
Exercised (in dollars per share)	93.04
Canceled/Forfeited (in dollars per share)	179.87
End of year (in dollars per share)	161.26	$ 131.26
Exercisable at end of year(in dollars per share)	128.61
Vested and expected to vest at end of year (in dollars per share)	$ 143.96
Shares
Beginning of year (in shares)	1,256,000
Granted (in shares)	47,000
Exercised (in shares)	(302,000)
Canceled/Forfeited (in shares)	(27,000)
End of year (in shares)	974,000	1,256,000
Exercisable at end of year (in shares)	667,000
Vested and expected to vest at end of year (in shares)	957,000
Total Intrinsic Value of Shares
Total intrinsic value of awards, at end of year	$ 59
Total intrinsic value of awards, exercisable at end of year	51
Total intrinsic value of awards, vested and expected to best at end of year	$ 59
Average Remaining Life (Years)
Average remaining life, at end of year	6 years 3 months 18 days
Average remaining life, exercisable at year end	5 years 3 months 18 days
Average remaining life, vested and expected to vest at end of year	6 years 3 months 18 days