LEASES - Leasing Arrangements as Lessor (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) MW	Dec. 31, 2018 USD ($)
Other operating revenue
Lease revenue \| $	$ 12.1
Lease revenue \| $		$ 17.6
J. M. Shafer Generating Station
Megawatt usage \| MW	272
J. M. Shafer Generating Station \| Third Party
Megawatt usage \| MW	122

Amount of operating lease income from lease payments paid and payable to lessor. Excludes variable lease payments not included in measurement of lease receivable.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 6A
-URI http://asc.fasb.org/extlink&oid=118952077&loc=SL77927221-108306

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119202524&loc=SL77919372-209981

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