SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Details) $ in Thousands, T in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2024 USD ($) contract T	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($) customer contract segment T $ / MWh	Dec. 31, 2023 USD ($) customer	Nov. 30, 2023 USD ($)	May 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Number of reportable segments | segment					2
Accounts Receivable, Allowance for Credit Loss, Ending Balance	$ 0				$ 0	$ 0
Asset impairment	215,100				215,136	0
Contract with Customer, Liability	146,719				146,719	113,741			$ 207,695
Contract value	52,100
Other operating and maintenance costs					118,364	199,855
Regulatory Asset	$ 30,800				30,800
Undiscounted Asset Retirement Obligations					$ 26,100	$ 26,600
Number of coal contracts | contract	0				0
Coal Supply Commitment (US Ton) | T	8.4				8.4
Year Supply Commitments End					2028
Sunrise Energy, LLC
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Percentage of equity method investment	50.00%				50.00%
Minimum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)					2 years
Minimum | Measurement Input, Discount Rate
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
ARO measurement input	7				7
Maximum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Coal Supply Commitment (US Ton) | T	9.2				9.2
Share-based Compensation Arrangement by Share-based Payment Award, Award Vesting Period (Year)					4 years
Maximum | Measurement Input, Discount Rate
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
ARO measurement input	10				10
Revision of Prior Period, Reclassification, Adjustment [Member]
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Other operating and maintenance costs		$ (1,700)	$ (2,600)	$ (2,100)
Surface And Underground Mining Equipment | Minimum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Property, Plant and Equipment, Useful Life (Year)	3 years				3 years
Surface And Underground Mining Equipment | Maximum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Property, Plant and Equipment, Useful Life (Year)	15 years				15 years
Power Plant Equipment | Minimum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Property, Plant and Equipment, Useful Life (Year)	7 years				7 years
Power Plant Equipment | Maximum
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Property, Plant and Equipment, Useful Life (Year)	9 years				9 years
Revenue, Product and Service Benchmark | Customer Concentration Risk | Five Customers | Coal
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage						93.00%
Number Of Major Customers | customer						5
Revenue, Product and Service Benchmark | Customer Concentration Risk | Three Customers | Coal
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage					94.00%
Number Of Major Customers | customer					3
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Three Customers | Delivered energy
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage					89.00%
Number Of Major Customers | customer					3
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Three Customers | Energy Capacity
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage						91.00%
Number Of Major Customers | customer						3
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Four Customers | Energy Capacity
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage					88.00%
Number Of Major Customers | customer					4
Revenue from Contract with Customer Benchmark | Customer Concentration Risk | Hoosier | Delivered energy
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage						100.00%
Accounts Receivable | Customer Concentration Risk | Two Customer
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage						100.00%
Number Of Major Customers | customer						2
Accounts Receivable | Customer Concentration Risk | Three Customers
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage					100.00%
Number Of Major Customers | customer					3
Accounts Receivable | Customer Concentration Risk | Four Customers | Coal
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Concentration Risk, Percentage					98.00%	85.00%
Number Of Major Customers | customer					4	4
Coal Purchase Agreement
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Contract with Customer, Asset, Amortized Expense						$ 19,600
Coal Purchase Agreement | Operating Expense
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Contract with Customer, Asset, Amortized Expense						30,700
Power Purchase Agreement | Hoosier
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Additional Obligations	$ 186,600				$ 186,600
Long-term Purchase Commitment, Minimum Quantity Required, Price Per Megawatt Hour | $ / MWh					56
Capacity payment reductions							$ 7,500	$ 7,500
Contract with Customer, Liability	43,500				$ 43,500
Contract With Customer Liability Amortization Expense					47,100	$ 70,500
Capacity Payment Reduction Agreement
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Contract with Customer, Liability	42,000				42,000
Physically Delivered Prepaid Power Contract
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Contract with Customer, Liability	61,200				61,200
Contract value					60,000
Contract with Customer, Liability, Implied Interest	$ 1,200				$ 1,200