SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Electric Plant and Depreciation (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ELECTRIC PLANT AND DEPRECIATION:
Interest rates charged during construction (as a percent)	4.70%	4.70%	4.70%
Interest capitalized during construction	$ 8.7	$ 8.6	$ 11.0

Discloses the interest rate related to debt for plant construction for an electric utility using an intermediary for each period for which an income statement is presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 810
-Section S99
-Paragraph 2
-Subparagraph (SAB Topic 10.A)
-URI http://asc.fasb.org/extlink&oid=116722093&loc=d3e661063-123039

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentInterestRateOnIntermediaryDebtForElectricUtilityPlantConstruction
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration