PROPERTY, PLANT AND EQUIPMENT - Other Plant (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Other Plant Assets
Other plant assets	$ 963,871	$ 952,318
Less allowances for depreciation, amortization and depletion	(743,752)	(711,896)
Net other plant	220,119	240,422
Mine assets
Other Plant Assets
Less allowances for depreciation, amortization and depletion	(216,142)	(184,239)
Net other plant	198,504	218,650
Mine assets | Colowyo Mine
Other Plant Assets
Other plant assets	408,359	396,441
Mine assets | New Horizon Mine
Other Plant Assets
Other plant assets	6,287	6,448
Non-utility assets
Other Plant Assets
Other plant assets	549,225	549,430
Less allowances for depreciation, amortization and depletion	(527,610)	(527,658)
Net other plant	$ 21,615	$ 21,772
Elk Ridge
Property, Plant and Equipment [Line Items]
Ownership interest held by parent (as a percent)	100.00%