SEGMENTS OF BUSINESS - Revenuer Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024
SEGMENTS OF BUSINESS
Other Operating Revenue	$ 1,659	$ 1,263
Total sales and operating revenues	117,787	111,574
Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	72,136	48,908
Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	13,807	11,773
Coal Sales
SEGMENTS OF BUSINESS
Operating Revenues	30,185	49,630
Coal Sales | Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	30,185	49,630
Electric Operations
SEGMENTS OF BUSINESS
Operating Revenues	85,943	60,681
Electric Operations | Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	72,136	48,908
Electric Operations | Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	13,807	11,773
Coal Operations
SEGMENTS OF BUSINESS
Operating Revenues	30,185	49,630
Operating Segment | Electric Operations
SEGMENTS OF BUSINESS
Other Operating Revenue	87	157
Total sales and operating revenues	86,030	60,838
Operating Segment | Electric Operations | Delivered Energy
SEGMENTS OF BUSINESS
Operating Revenues	72,136	48,908
Operating Segment | Electric Operations | Capacity Revenue
SEGMENTS OF BUSINESS
Operating Revenues	13,807	11,773
Operating Segment | Coal Operations
SEGMENTS OF BUSINESS
Other Operating Revenue	1,324	810
Total sales and operating revenues	56,098	66,846
Operating Segment | Coal Operations | Coal Sales
SEGMENTS OF BUSINESS
Operating Revenues	54,774	66,036
Operating Segment | Coal Operations | Coal Sales | Third-Party
SEGMENTS OF BUSINESS
Operating Revenues	30,185	49,630
Operating Segment | Coal Operations | Coal Sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	24,589	16,406
Corporate and Other Eliminations
SEGMENTS OF BUSINESS
Other Operating Revenue	248	296
Total sales and operating revenues	(24,341)	(16,110)
Corporate and Other Eliminations | Coal Sales | Intercompany
SEGMENTS OF BUSINESS
Operating Revenues	$ (24,589)	$ (16,406)