Transactions with Controlled Companies - NMMB, Inc. (Details) - USD ($) $ in Thousands	12 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2023
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income	$ 671,109	$ 770,312	$ 760,785
Dividend Income	19,378	11,953	8,405
Net realized gains (losses)	(518,682)	(417,443)	(41,046)
Interest Receivable	$ 25,144	$ 26,936
NMMB, Inc. \| NMMB Holdings, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	92.77%	92.77%
NMMB, Inc. \| Refuel Agency, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Armed Forces Communications, Inc. \| NMMB, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Reportable Legal Entities \| Subsidiaries \| NMMB, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income	$ 4,039	$ 4,255	3,754
Dividend Income	0	657	2,510
Net realized gains (losses)	6,366	1,040	(2,510)
Interest Receivable	11	35
Other Receivables	10	1
Reportable Legal Entities \| Subsidiaries \| NMMB, Inc. \| Prospect Administration
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Managerial Assistance	400	400	400
Reimbursement of Legal, Tax, etc	$ 9	$ 1	$ 4
Reportable Legal Entities \| Subsidiaries \| NMMB Holdings, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%

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