SEGMENTS OF BUSINESS - Revenuer Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024
SEGMENTS OF BUSINESS
Other operating revenue	$ 10,074	$ 5,184
Total sales and operating revenues	469,466	404,159
Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	252,644	203,434
Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	58,093
Coal sales
SEGMENTS OF BUSINESS
Operating Revenues	148,655	137,448
Coal sales | Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	148,655	137,448
Electric Operations
SEGMENTS OF BUSINESS
Operating Revenues	310,737	261,527
Electric Operations | Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	252,644	203,434
Electric Operations | Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	58,093
Coal Operations
SEGMENTS OF BUSINESS
Operating Revenues	148,655	137,448
Operating Segment | Electric Operations
SEGMENTS OF BUSINESS
Other operating revenue	3,534	946
Total sales and operating revenues	314,271	262,473
Operating Segment | Electric Operations | Delivered energy
SEGMENTS OF BUSINESS
Operating Revenues	252,644	203,434
Operating Segment | Electric Operations | Accredited capacity revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	58,093
Operating Segment | Coal Operations
SEGMENTS OF BUSINESS
Other operating revenue	5,373	2,559
Total sales and operating revenues	226,381	205,084
Operating Segment | Coal Operations | Coal sales
SEGMENTS OF BUSINESS
Operating Revenues	221,008	202,525
Operating Segment | Coal Operations | Coal sales | Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	148,655	137,448
Operating Segment | Coal Operations | Coal sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	72,353	65,077
Corporate and Other Eliminations
SEGMENTS OF BUSINESS
Other operating revenue	1,167	1,679
Total sales and operating revenues	(71,186)	(63,398)
Corporate and Other Eliminations | Coal sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	$ (72,353)	$ (65,077)