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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		6 Months Ended
	Jun. 30, 2017 USD ($) MMBTU MWh gal	Jun. 30, 2016 USD ($) MMBTU MWh gal	Jun. 30, 2017 USD ($) MMBTU MWh gal	Jun. 30, 2016 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 3.1	$ (19.4)	$ 5.3	$ (51.0)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 1.3	$ (20.0)	$ 1.0	$ (53.5)
Notional Sales Volumes
Notional sales volumes | MMBTU	25.2	32.7	59.3	82.8
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.4)	$ (1.0)	$ (0.9)	$ (2.1)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	4.9	3.6	9.8	6.6
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 2.2	$ 1.6	$ 5.2	$ 4.6
Notional Sales Volumes
Notional sales volumes | MWh	9.4	7.4	18.6	15.0

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=64837297&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=64837297&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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