PROPERTY, PLANT AND EQUIPMENT - Other Plant (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Other Plant Assets
Other plant assets	$ 409,051	$ 384,650
Less allowances for depreciation, amortization and depletion	(113,607)	(110,939)
Net other plant	295,444	273,711
Mine assets
Other Plant Assets
Less allowances for depreciation, amortization and depletion	(106,337)	(104,031)
Net other plant	290,070	268,242
Mine assets | Colowyo Mine
Other Plant Assets
Other plant assets	356,612	326,838
Mine assets | New Horizon Mine
Other Plant Assets
Other plant assets	38,949	44,589
Mine assets | Fort Union Mine
Other Plant Assets
Other plant assets	846	846
Non-utility assets
Other Plant Assets
Other plant assets	12,644	12,377
Less allowances for depreciation, amortization and depletion	(7,270)	(6,908)
Net other plant	$ 5,374	$ 5,469
Elk Ridge
Other Plant Assets
Ownership interest held by parent (as a percent)	100.00%
Fort Union Mine
Other Plant Assets
Ownership interest in land and rights to the mine (as a percent)	50.00%