Equity-Based Compensation - Additional Information (Details) - USD ($) $ in Thousands		12 Months Ended
	Jun. 25, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	May 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense		$ 10,452	$ 6,124	$ 1,567
2018 Equity Incentive Award Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available for grant under the plan (shares)		205,151			3,500,000
Additional Class A common shares added at the beginning of calendar year as percentage of common stock outstanding at end of previous year (percent)					4.00%
TRA non-participation compensatory shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity-based compensation expense	$ 741	$ 0	$ 0	$ 741