SEGMENTS OF BUSINESS - Summary of reportable segments operations (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023
Segment Reporting Information [Line Items]
Operating revenues		$ 105,044	$ 165,768	$ 310,175	$ 515,296
Operating Expense		100,332	141,965	310,016	440,016
Income (loss) from operations		4,712	23,803	159	75,280
Depreciation, Depletion and Amortization		13,838	16,230	42,930	51,375
Assets		579,730	585,186	579,730	585,186	$ 589,780
Capital Expenditures		11,562	18,136	39,606	48,746
Other revenues		1,667	945	4,221	3,888
Electric Operations
Segment Reporting Information [Line Items]
Operating revenues	[1]	71,902	67,544	192,379	231,141
Operating Expense		52,547	70,220	150,989	205,856
Income (loss) from operations		19,355	(2,676)	41,390	25,285
Depreciation, Depletion and Amortization		4,802	4,695	14,197	14,045
Assets		217,826	209,455	217,826	209,455
Capital Expenditures		4,602	6,566	16,121	10,092
Amortization of contract liability		11,306	10,281	36,174	63,183
Electric Operations | Capacity revenue
Segment Reporting Information [Line Items]
Operating revenues		15,860	13,012	44,506	46,137
Electric Operations | Delivered energy
Segment Reporting Information [Line Items]
Other revenues		44,549	44,110	111,181	121,492
Electric Operations | Other operating revenue
Segment Reporting Information [Line Items]
Other revenues		187	141	518	329
Coal Operations
Segment Reporting Information [Line Items]
Operating revenues		49,331	134,896	162,630	343,267
Operating Expense		61,510	110,132	197,587	279,052
Income (loss) from operations		(12,179)	24,764	(34,957)	64,215
Depreciation, Depletion and Amortization		9,013	11,508	28,671	37,249
Assets		356,252	375,682	356,252	375,682
Capital Expenditures		6,810	11,570	23,002	38,654
Corporate and other and eliminations
Segment Reporting Information [Line Items]
Operating revenues		(16,189)	(36,672)	(44,834)	(59,112)
Operating Expense		(13,725)	(38,387)	(38,560)	(44,892)
Income (loss) from operations		(2,464)	1,715	(6,274)	(14,220)
Depreciation, Depletion and Amortization		23	27	62	81
Assets		5,652	49	5,652	49
Capital Expenditures		$ 150	$ 0	$ 483	$ 0

[1] Electric operations revenue as of each period presented were comprised of the components noted below (in thousands):