REVENUE - Remaining performance obligations (Details) - Operating Segment $ in Thousands	Dec. 31, 2025 USD ($)
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 866,940
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 389,540
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 335,540
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 124,530
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 17,330
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 389,730
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 175,880
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 142,290
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 57,700
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Delivered energy | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 13,860
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Accredited Energy Capacity
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 153,740
Electric Operations | Accredited Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 61,540
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Accredited Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 51,400
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Accredited Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 37,330
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Electric Operations | Accredited Energy Capacity | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 3,470
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 323,470
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 152,120
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 141,850
Unsatisfied or partially unsatisfied performance obligation, period	1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction [Line Items]
Revenue expected to be recognized	$ 29,500
Unsatisfied or partially unsatisfied performance obligation, period	1 year