PROPERTY, PLANT AND EQUIPMENT - Other Plant (Details) - USD ($) $ in Thousands	Jun. 30, 2024	Dec. 31, 2023
Other Plant Assets
Other plant assets	$ 950,982	$ 952,318
Less allowances for depreciation, amortization and depletion	(726,882)	(711,896)
Net other plant	224,100	240,422
Mine assets
Other Plant Assets
Less allowances for depreciation, amortization and depletion	(199,278)	(184,239)
Net other plant	202,480	218,650
Mine assets | Colowyo Mine
Other Plant Assets
Other plant assets	395,438	396,441
Mine assets | New Horizon Mine
Other Plant Assets
Other plant assets	6,320	6,448
Non-utility assets
Other Plant Assets
Other plant assets	549,224	549,430
Less allowances for depreciation, amortization and depletion	(527,604)	(527,658)
Net other plant	$ 21,620	$ 21,772
Elk Ridge
Property, Plant and Equipment [Line Items]
Ownership interest held by parent (as a percent)	100.00%