RELATED PARTY TRANSACTIONS - Related Party Revenue and Cost of Sales (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Related Party Transaction [Line Items]
Total revenue	$ 48,684	$ 95,700
Cost of sales (excluding depreciation, depletion and amortization)	35,594	24,216
Related party
Related Party Transaction [Line Items]
Cost of sales (excluding depreciation, depletion and amortization)	19,232	22,709
Rare earth concentrate
Related Party Transaction [Line Items]
Total revenue	40,076	95,666
Rare earth concentrate | Related party
Related Party Transaction [Line Items]
Total revenue	40,076	89,054
Other rare earth products
Related Party Transaction [Line Items]
Total revenue	281	34
NdPr oxide and metal (including related party)
Related Party Transaction [Line Items]
Total revenue	8,327	0
NdPr oxide and metal (including related party) | Related party
Related Party Transaction [Line Items]
Total revenue	$ 546	$ 0