SEGMENTS OF BUSINESS - Revenuer Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
SEGMENTS OF BUSINESS
Other Operating Revenue	$ 5,419	$ 5,025
Total sales and operating revenues	404,394	634,878
Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	203,434	211,772
Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	56,155
Coal Sales
SEGMENTS OF BUSINESS
Operating Revenues	137,448	361,926
Coal Sales | Third Party
SEGMENTS OF BUSINESS
Operating Revenues	137,448	361,926
Electric Operations
SEGMENTS OF BUSINESS
Operating Revenues	261,527	267,927
Electric Operations | Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	203,434	211,772
Electric Operations | Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	56,155
Coal Operations
SEGMENTS OF BUSINESS
Operating Revenues	137,448	361,926
Operating Segment | Electric Operations
SEGMENTS OF BUSINESS
Other Operating Revenue	982	414
Total sales and operating revenues	262,509	268,341
Operating Segment | Electric Operations | Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	203,434	211,772
Operating Segment | Electric Operations | Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	58,093	56,155
Operating Segment | Coal Operations
SEGMENTS OF BUSINESS
Other Operating Revenue	2,756	2,936
Total sales and operating revenues	205,281	435,824
Operating Segment | Coal Operations | Coal Sales
SEGMENTS OF BUSINESS
Operating Revenues	202,525	432,888
Operating Segment | Coal Operations | Coal Sales | Third Party
SEGMENTS OF BUSINESS
Operating Revenues	137,448	361,926
Operating Segment | Coal Operations | Coal Sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	65,077	70,962
Corporate and Other and Eliminations
SEGMENTS OF BUSINESS
Other Operating Revenue	1,681	1,675
Total sales and operating revenues	(63,396)	(69,287)
Corporate and Other and Eliminations | Coal Sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	$ (65,077)	$ (70,962)