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DERIVATIVE INSTRUMENTS - GAINS (LOSSES) AND NOTIONAL VOLUMES (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) MMBTU MWh gal	Sep. 30, 2016 USD ($) MMBTU MWh gal	Sep. 30, 2017 USD ($) MMBTU MWh gal	Sep. 30, 2016 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 1.6	$ 3.3	$ 6.9	$ (47.7)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (2.1)	$ (3.4)	$ (1.1)	$ (56.9)
Notional Sales Volumes
Notional sales volumes | MMBTU	24.9	30.5	84.2	113.3
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.5)	$ (0.4)	$ (1.4)	$ (2.5)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	4.4	4.3	14.2	10.9
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 4.2	$ 7.1	$ 9.4	$ 11.7
Notional Sales Volumes
Notional sales volumes | MWh	9.4	9.9	28.0	24.9

X

- 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

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- Definition

Nominal energy measure used to calculate payments on a derivative instrument.

+ References

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- Definition

Nominal volume used to calculate payments on a derivative instrument.

+ References

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