Equity-Based Compensation - Schedule of Weighted-Average Assumptions Used in Valuation of Unit Awards, Restricted Stock Unit Awards Were Converted and Stock Option Awards Granted or Modified (Details)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	1.56%	2.85%	2.77%
Volatility factor	43.51%	38.85%	26.93%
Expected term (in years)	6 years 3 months 18 days	6 years 3 months 18 days	6 years