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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, 2018 USD ($) MWh MMBTU gal | Dec. 31, 2017 USD ($) MWh MMBTU gal | Dec. 31, 2016 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 42.1 | $ 4.7 | $ (49.5)
Natural gas contracts |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 24.6 | $ (8.0) | $ (59.6)
Notional Sales Volumes |  |  |
Notional sales volumes (Dth or MWh) | MMBTU | 173.2 | 123.1 | 151.1
Petroleum products contracts |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 1.6 | $ (1.3) | $ (3.2)
Notional Sales Volumes |  |  |
Notional sales volumes (gallons) | gal | 6.0 | 18.0 | 14.7
FTRs |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 15.9 | $ 14.0 | $ 13.3
Notional Sales Volumes |  |  |
Notional sales volumes (Dth or MWh) | MWh | 30.5 | 36.2 | 33.7

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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 + Details
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