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DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) MMBTU MWh	Mar. 31, 2019 USD ($) MMBTU MWh
Realized gains (losses)
Gains (losses)	$ (23.3)	$ 1.8
Natural gas contracts
Realized gains (losses)
Gains (losses)	$ (24.7)	$ (0.5)
Notional sales volumes
Notional sales volumes | MMBTU	58.4	56.1
FTRs
Realized gains (losses)
Gains (losses)	$ 1.4	$ 2.3
Notional sales volumes
Notional sales volumes | MWh	7.2	8.1

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- Definition

Amount of increase (decrease) in the fair value of derivatives recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5618551-113959

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- References

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- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

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