Transactions with Controlled Companies - First Tower Finance Company LLC (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2022
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income	$ 174,318	$ 146,271
Structuring Fee	4,627	11,962
Total Other Income	25,455	21,936
Interest Receivable	18,781		$ 12,925
Other Receivables	$ 1,309		$ 745
First Tower Holdings of Delaware LLC \| First Tower Finance Company LLC
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	78.06%		80.03%
First Tower Finance Company LLC \| First Tower, LLC
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%
Reportable Legal Entities \| Subsidiaries \| First Tower Holdings of Delaware LLC
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%
Reportable Legal Entities \| Subsidiaries \| First Tower Holdings of Delaware LLC
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income Capitalized as PIK	$ 9,576	3,777
Repayment of Loan Receivable	0	308
Interest Receivable	224		$ 218
Other Receivables	6		$ 6
Reportable Legal Entities \| Subsidiaries \| First Tower Finance Company LLC
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income	20,235	18,338
Structuring Fee	0	7,234
Total Other Income	0	7,234
Reportable Legal Entities \| Subsidiaries \| First Tower Finance Company LLC \| Prospect Administration
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Managerial Assistance	$ 0	$ 600

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