v3.26.1
REVENUE - Remaining performance obligations (Details) - Operating Segment
$ in Thousands
Jun. 30, 2026
USD ($)
REVENUE  
Revenue expected to be recognized $ 1,842,510
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-07-01  
REVENUE  
Revenue expected to be recognized $ 225,680
Unsatisfied or partially unsatisfied performance obligation, period 6 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01  
REVENUE  
Revenue expected to be recognized $ 366,210
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01  
REVENUE  
Revenue expected to be recognized $ 207,000
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01  
REVENUE  
Revenue expected to be recognized $ 131,300
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01  
REVENUE  
Revenue expected to be recognized $ 87,540
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01  
REVENUE  
Revenue expected to be recognized $ 824,780
Unsatisfied or partially unsatisfied performance obligation, period
Electric Operations | Delivered energy revenue  
REVENUE  
Revenue expected to be recognized $ 390,090
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-07-01  
REVENUE  
Revenue expected to be recognized $ 114,350
Unsatisfied or partially unsatisfied performance obligation, period 6 months
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01  
REVENUE  
Revenue expected to be recognized $ 160,260
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01  
REVENUE  
Revenue expected to be recognized $ 86,550
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01  
REVENUE  
Revenue expected to be recognized $ 28,930
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01  
REVENUE  
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Delivered energy revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01  
REVENUE  
Unsatisfied or partially unsatisfied performance obligation, period
Electric Operations | Accredited capacity revenue  
REVENUE  
Revenue expected to be recognized $ 1,215,940
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-07-01  
REVENUE  
Revenue expected to be recognized $ 34,990
Unsatisfied or partially unsatisfied performance obligation, period 6 months
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01  
REVENUE  
Revenue expected to be recognized $ 75,310
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2028-01-01  
REVENUE  
Revenue expected to be recognized $ 90,950
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01  
REVENUE  
Revenue expected to be recognized $ 102,370
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01  
REVENUE  
Revenue expected to be recognized $ 87,540
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Electric Operations | Accredited capacity revenue | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01  
REVENUE  
Revenue expected to be recognized $ 824,780
Unsatisfied or partially unsatisfied performance obligation, period
Coal Operations  
REVENUE  
Revenue expected to be recognized $ 236,480
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2026-07-01  
REVENUE  
Revenue expected to be recognized $ 76,340
Unsatisfied or partially unsatisfied performance obligation, period 6 months
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2027-01-01  
REVENUE  
Revenue expected to be recognized $ 130,640
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  
Revenue expected to be recognized $ 29,500
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2029-01-01  
REVENUE  
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2030-01-01  
REVENUE  
Unsatisfied or partially unsatisfied performance obligation, period 1 year
Coal Operations | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2031-01-01  
REVENUE  
Unsatisfied or partially unsatisfied performance obligation, period