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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
	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

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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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