v3.26.1
Revenue (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers $ 841,945 $ 777,112
Other revenue (742) 1,500
Total operating revenue 841,203 778,612
Sales of electricity to members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 600,778 555,678
Sales of electricity to non-members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 68,804 59,317
Synthetic natural gas    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 42,056 39,888
Fertilizer and DEF products    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 55,900 51,411
Other byproducts    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 16,895 18,689
Lignite coal    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 48,096 42,370
Miscellaneous    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 9,416 9,759
Intersegment Eliminations    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers (30,966) (28,604)
Other revenue 0 0
Total operating revenue (30,966) (28,604)
Intersegment Eliminations | Sales of electricity to members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Intersegment Eliminations | Sales of electricity to non-members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Intersegment Eliminations | Synthetic natural gas    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Intersegment Eliminations | Fertilizer and DEF products    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Intersegment Eliminations | Other byproducts    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Intersegment Eliminations | Lignite coal    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers (30,966) (28,604)
Intersegment Eliminations | Miscellaneous    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Electric Utility | Operating Segments    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 671,589 617,572
Other revenue 0 861
Total operating revenue 671,589 618,433
Electric Utility | Operating Segments | Sales of electricity to members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 600,778 555,678
Electric Utility | Operating Segments | Sales of electricity to non-members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 68,804 59,317
Electric Utility | Operating Segments | Synthetic natural gas    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Electric Utility | Operating Segments | Fertilizer and DEF products    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Electric Utility | Operating Segments | Other byproducts    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Electric Utility | Operating Segments | Lignite coal    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Electric Utility | Operating Segments | Miscellaneous    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 2,007 2,577
Gasification | Operating Segments    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 115,195 110,317
Other revenue (742) 639
Total operating revenue 114,453 110,956
Gasification | Operating Segments | Sales of electricity to members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Gasification | Operating Segments | Sales of electricity to non-members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Gasification | Operating Segments | Synthetic natural gas    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 42,056 39,888
Gasification | Operating Segments | Fertilizer and DEF products    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 55,900 51,411
Gasification | Operating Segments | Other byproducts    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 16,895 18,689
Gasification | Operating Segments | Lignite coal    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Gasification | Operating Segments | Miscellaneous    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 344 329
Coal and Limestone Operations | Operating Segments    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 86,127 77,827
Other revenue 0 0
Total operating revenue 86,127 77,827
Coal and Limestone Operations | Operating Segments | Sales of electricity to members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Coal and Limestone Operations | Operating Segments | Sales of electricity to non-members    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Coal and Limestone Operations | Operating Segments | Synthetic natural gas    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Coal and Limestone Operations | Operating Segments | Fertilizer and DEF products    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Coal and Limestone Operations | Operating Segments | Other byproducts    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 0 0
Coal and Limestone Operations | Operating Segments | Lignite coal    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers 79,062 70,974
Coal and Limestone Operations | Operating Segments | Miscellaneous    
Disaggregation of Revenue [Line Items]    
Revenue from contracts with customers $ 7,065 $ 6,853