Shareholders’ Equity - Issuances/Repurchases (Details) shares in Thousands, $ in Thousands	12 Months Ended
	Sep. 30, 2021 USD ($) vesting_installment shares	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
General and administrative expense	$ 26,961	$ 26,514	$ 28,706
Total compensation and benefits expense	$ 136,191	$ 131,095	$ 130,619
Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) | shares	96,300	93,700	77,900
Managing Director | Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted (in shares) | shares	3	3	2,500
2016 Plan | Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of installments | vesting_installment	5
General and administrative expense	$ 771	$ 564	$ 784
Adjustment related to tax withholding for share based compensation	834	523	827
2016 Plan | Officers and Employees | Class A
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based payment arrangement, forfeited	22	49	33
Total compensation and benefits expense	$ 2,868	$ 2,916	$ 2,579