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Derivative Instruments - Gains (Losses) and Notional Volumes (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2020 USD ($) MWh MMBTU gal | Dec. 31, 2019 USD ($) MWh MMBTU gal | Dec. 31, 2018 USD ($) MMBTU MWh gal
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ (50.0) | $ (10.8) | $ 42.1
Natural gas contracts |  |  |
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ (54.1) | $ (27.1) | $ 24.6
Notional sales volumes |  |  |
Notional sales volumes | MMBTU | 188.6 | 183.9 | 173.2
Petroleum products contracts |  |  |
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ 0.0 | $ 0.0 | $ 1.6
Notional sales volumes |  |  |
Notional sales volumes (gallons) | gal | 0.0 | 0.0 | 6.0
FTRs |  |  |
Realized gains (losses) on derivatives |  |  |
Gains (losses) | $ 4.1 | $ 16.3 | $ 15.9
Notional sales volumes |  |  |
Notional sales volumes | MWh | 29.8 | 31.2 | 30.5

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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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