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Transactions with Controlled Companies - Valley Electric Company, Inc. (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2025	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2024
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income		$ 581,932	$ 671,109	$ 770,312
Dividend Income		46,967	19,378	11,953
Total Other Income		10,555	28,949	79,397
Interest Receivable		$ 17,350	25,144
Valley Electric Holdings II, Inc. \| Valley Electric Holdings I, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Valley Electric Company, Inc. \| Members of Valley Electric Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest		5.01%
Valley Electric Company, Inc. \| Valley Electric Holdings II, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	94.99%	94.99%
VE Company, Inc. \| Valley Electric Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Valley Electric Co. of Mt. Vernon, Inc. \| VE Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Reportable Legal Entities \| Subsidiaries \| Valley Electric Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income		$ 12,538	12,677	12,316
Dividend Income		10,924	0	0
Royalty, net profit and revenue interests		666	666	666
Total Other Income		666	666	666
Interest Income Capitalized as PIK		0	0	4,763
Interest Receivable		35	757
Other Receivables		9	9
Reportable Legal Entities \| Subsidiaries \| Valley Electric Company, Inc. \| Prospect Administration
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Managerial Assistance		600	600	600
Reimbursement of Legal, Tax, etc		$ 12	3	0
Reportable Legal Entities \| Subsidiaries \| Valley Electric Holdings I, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Ownership interest	100.00%	100.00%
Reportable Legal Entities \| Subsidiaries \| Valley Electric Company, Inc. \| Valley Electric Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income		$ 11,300	11,363	10,927
Reportable Legal Entities \| Subsidiaries \| Valley Electric Co. of Mt. Vernon, Inc. \| Valley Electric Company, Inc.
Subsidiary of Limited Liability Company or Limited Partnership [Line Items]
Interest Income		$ 1,238	$ 1,314	$ 1,389