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DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) MWh in Millions, MMBTU in Millions, $ in Millions | 3 Months Ended
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Mar. 31, 2021 USD ($) MMBTU MWh | Mar. 31, 2020 USD ($) MMBTU MWh
Realized gains (losses) |  |
Gains (losses) | $ (5.4) | $ (23.3)
Natural gas contracts |  |
Realized gains (losses) |  |
Gains (losses) | $ (7.5) | $ (24.7)
Notional sales volumes |  |
Notional sales volumes | MMBTU | 59.8 | 58.4
FTRs |  |
Realized gains (losses) |  |
Gains (losses) | $ 2.1 | $ 1.4
Notional sales volumes |  |
Notional sales volumes | MWh | 8.4 | 7.2

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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=121590274&loc=SL5618551-113959 + Details
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