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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, 2017 USD ($) MWh MMBTU gal | Dec. 31, 2016 USD ($) MWh MMBTU gal | Dec. 31, 2015 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 4.7 | $ (49.5) | $ (45.7)
Natural gas contracts |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ (8.0) | $ (59.6) | $ (50.5)
Notional Sales Volumes |  |  |
Notional sales volumes (Dth or MWh) | MMBTU | 123.1 | 151.1 | 86.2
Petroleum products contracts |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ (1.3) | $ (3.2) | $ (1.9)
Notional Sales Volumes |  |  |
Notional sales volumes (gallons) | gal | 18.0 | 14.7 | 7.8
FTRs |  |  |
Realized Gain (Loss) on Derivatives, Net |  |  |
Gains (Losses) | $ 14.0 | $ 13.3 | $ 6.7
Notional Sales Volumes |  |  |
Notional sales volumes (Dth or MWh) | MWh | 36.2 | 33.7 | 27.3

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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/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4A -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5618551-113959 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (a),(c),(d),(e) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959 + Details
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- Definition Nominal volume used to calculate payments on a derivative instrument. + References No definition available. + Details
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