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Derivative Instruments - Gains (Losses) and Notional Volumes (Details) MWh in Millions, MMBTU in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2021 USD ($) MMBTU MWh | Dec. 31, 2020 USD ($) MMBTU MWh | Dec. 31, 2019 USD ($) MWh MMBTU
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ 154.2 | $ (50.0) | $ (10.8)
Natural gas contracts |  |  |
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ 136.5 | $ (54.1) | $ (27.1)
Notional sales volumes |  |  |
Notional sales volumes | MMBTU | 197.6 | 188.6 | 183.9
FTRs |  |  |
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ 17.7 | $ 4.1 | $ 16.3
Notional sales volumes |  |  |
Notional sales volumes | MWh | 28.2 | 29.8 | 31.2

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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=125515794&loc=SL5618551-113959 + Details
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- Definition Nominal energy measure used to calculate payments on a derivative instrument. + References No definition available. + Details
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