COMMITMENTS AND CONTINGENCIES - Purchase Commitments (Details) $ in Millions	Dec. 31, 2022 USD ($)
Commitments for external coal purchases
Contractual Commitments
Commitments to purchase coal	$ 0.0
Commitments related to planned capital projects
Contractual Commitments
Contractual amount	$ 147.7

Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

No definition available.

Name:	us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant