PROPERTY, PLANT AND EQUIPMENT - Jointly Owned Facilities (Details) $ in Thousands |
Sep. 30, 2026
USD ($)
|
|---|---|
| Jointly Owned Utility Plant Interests [Line Items] | |
| Electric Plant in Service | $ 938,879 |
| Accumulated Depreciation | 751,786 |
| Construction Work In Progress | $ 3,616 |
| Yampa Project - Craig Generating Station Units 1 and 2 | |
| Jointly Owned Utility Plant Interests [Line Items] | |
| Tri-State Share (as a percent) | 24.00% |
| Electric Plant in Service | $ 392,523 |
| Accumulated Depreciation | 383,905 |
| Construction Work In Progress | $ 0 |
| MBPP - Laramie River Station | |
| Jointly Owned Utility Plant Interests [Line Items] | |
| Tri-State Share (as a percent) | 28.50% |
| Electric Plant in Service | $ 546,356 |
| Accumulated Depreciation | 367,881 |
| Construction Work In Progress | $ 3,616 |