SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Inventories (Details) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
INVENTORIES:
Coal inventory	$ 54,979,000	$ 34,723,000
Owned Generating Stations
INVENTORIES:
Coal inventory	29,000,000	$ 3,700,000
LIFO inventory liquidation	$ 0

Amount of inventory reserves for last-in first-out (LIFO) and other inventory valuation methods.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(6))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 330
-SubTopic 10
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147483080/330-10-50-1

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