SEGMENTS OF BUSINESS - Revenue Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
SEGMENTS OF BUSINESS
Other operating revenue	$ 1,395	$ 4,702	$ 3,026	$ 6,298
Total sales and operating revenues	101,505	102,825	205,993	220,549
Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	40,901	44,132	93,144	116,268
Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	18,608	15,844	34,142	29,651
Coal sales
SEGMENTS OF BUSINESS
Operating Revenues	40,601	38,147	75,681	68,332
Coal sales \| Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	40,601	38,147	75,681	68,332
Electric Operations
SEGMENTS OF BUSINESS
Operating Revenues	59,509	59,976	127,286	145,919
Electric Operations \| Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	40,901	44,132	93,144	116,268
Electric Operations \| Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	18,608	15,844	34,142	29,651
Coal Operations
SEGMENTS OF BUSINESS
Operating Revenues	40,601	38,147	75,681	68,332
Operating Segment \| Electric Operations
SEGMENTS OF BUSINESS
Operating Revenues	59,509	59,976	127,286	145,919
Other operating revenue	231	3,115	368	3,202
Total sales and operating revenues	59,740	63,091	127,654	149,121
Operating Segment \| Electric Operations \| Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	40,901	44,132	93,144	116,268
Operating Segment \| Electric Operations \| Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	18,608	15,844	34,142	29,651
Operating Segment \| Coal Operations
SEGMENTS OF BUSINESS
Operating Revenues	50,874	45,529	97,286	100,303
Other operating revenue	788	1,363	1,928	2,624
Total sales and operating revenues	51,662	46,892	99,214	102,927
Operating Segment \| Coal Operations \| Coal sales
SEGMENTS OF BUSINESS
Operating Revenues	50,874	45,529	97,286	100,303
Operating Segment \| Coal Operations \| Coal sales \| Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	40,601	38,147	75,681	68,332
Operating Segment \| Coal Operations \| Coal sales \| Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	10,273	7,382	21,605	31,971
Corporate and Other Eliminations
SEGMENTS OF BUSINESS
Other operating revenue	376	224	730	472
Total sales and operating revenues	(9,897)	(7,158)	(20,875)	(31,499)
Corporate and Other Eliminations \| Coal sales \| Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	$ (10,273)	$ (7,382)	$ (21,605)	$ (31,971)

The total amount of other operating income, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operation.

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