SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2024 USD ($)	Dec. 31, 2025 USD ($) segment	Dec. 31, 2024 USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of reportable segments | segment		2
Accounts receivable charges for credit losses	$ 0	$ 0	$ 0
Total deposits		13,600
Business interruption insurance settlements		12,900
Amount of cash paid for the settlement of business interruption insurance		0	0
Asset impairment	$ 215,100	0	215,136
Regulatory asset		30,900
Undiscounted asset retirement obligation		25,300	26,100
Coal Operations | Operating Segment
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Asset impairment		$ 0	$ 215,136
Minimum | Measurement Input, Discount Rate
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
ARO measurement input		7
Maximum | Measurement Input, Discount Rate
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
ARO measurement input		10
Power Plant Equipment | Minimum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Estimated useful life of the assets (in years)		7 years
Power Plant Equipment | Maximum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Estimated useful life of the assets (in years)		9 years
Surface and Underground Mining Equipment | Minimum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Estimated useful life of the assets (in years)		3 years
Surface and Underground Mining Equipment | Maximum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Estimated useful life of the assets (in years)		15 years
Sunrise Energy, LLC
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Percentage of equity method investment		50.00%