v3.26.3
SEGMENTS OF BUSINESS - Revenue Reconciliation (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2026
Sep. 30, 2025
Sep. 30, 2026
Sep. 30, 2025
SEGMENTS OF BUSINESS        
Other operating revenue $ 1,276 $ 2,066 $ 4,302 $ 8,364
Total sales and operating revenues 130,250 146,557 336,243 367,106
Delivered energy (including contract liability amortization)        
SEGMENTS OF BUSINESS        
Operating Revenues 74,169 77,776 167,313 194,044
Accredited capacity revenue        
SEGMENTS OF BUSINESS        
Operating Revenues 18,677 15,459 52,819 45,110
Coal sales        
SEGMENTS OF BUSINESS        
Operating Revenues 36,128 51,256 111,809 119,588
Coal sales | Third-Party        
SEGMENTS OF BUSINESS        
Operating Revenues 36,128 51,256 111,809 119,588
Coal sales | Intercompany        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Electric Operations        
SEGMENTS OF BUSINESS        
Operating Revenues 92,846 93,235 220,132 239,154
Electric Operations | Delivered energy (including contract liability amortization)        
SEGMENTS OF BUSINESS        
Operating Revenues 74,169 77,776 167,313 194,044
Electric Operations | Accredited capacity revenue        
SEGMENTS OF BUSINESS        
Operating Revenues 18,677 15,459 52,819 45,110
Coal Operations        
SEGMENTS OF BUSINESS        
Operating Revenues 36,128 51,256 111,809 119,588
Operating Segment | Electric Operations        
SEGMENTS OF BUSINESS        
Operating Revenues 92,846 93,235 220,132 239,154
Other operating revenue 149 170 517 3,372
Total sales and operating revenues 92,995 93,405 220,649 242,526
Operating Segment | Electric Operations | Delivered energy (including contract liability amortization)        
SEGMENTS OF BUSINESS        
Operating Revenues 74,169 77,776 167,313 194,044
Operating Segment | Electric Operations | Accredited capacity revenue        
SEGMENTS OF BUSINESS        
Operating Revenues 18,677 15,459 52,819 45,110
Operating Segment | Electric Operations | Coal sales        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Operating Segment | Electric Operations | Coal sales | Third-Party        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Operating Segment | Electric Operations | Coal sales | Intercompany        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Operating Segment | Coal Operations        
SEGMENTS OF BUSINESS        
Operating Revenues 50,631 68,814 147,917 169,117
Other operating revenue 696 1,582 2,624 4,206
Total sales and operating revenues 51,327 70,396 150,541 173,323
Operating Segment | Coal Operations | Delivered energy (including contract liability amortization)        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Operating Segment | Coal Operations | Accredited capacity revenue        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Operating Segment | Coal Operations | Coal sales        
SEGMENTS OF BUSINESS        
Operating Revenues 50,631 68,814 147,917 169,117
Operating Segment | Coal Operations | Coal sales | Third-Party        
SEGMENTS OF BUSINESS        
Operating Revenues 36,128 51,256 111,809 119,588
Operating Segment | Coal Operations | Coal sales | Intercompany        
SEGMENTS OF BUSINESS        
Operating Revenues 14,503 17,558 36,108 49,529
Corporate and Other and Eliminations        
SEGMENTS OF BUSINESS        
Other operating revenue 431 314 1,161 786
Total sales and operating revenues (14,072) (17,244) (34,947) (48,743)
Corporate and Other and Eliminations | Delivered energy (including contract liability amortization)        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Corporate and Other and Eliminations | Accredited capacity revenue        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Corporate and Other and Eliminations | Coal sales | Third-Party        
SEGMENTS OF BUSINESS        
Operating Revenues 0 0 0 0
Corporate and Other and Eliminations | Coal sales | Intercompany        
SEGMENTS OF BUSINESS        
Operating Revenues $ (14,503) $ (17,558) $ (36,108) $ (49,529)