Derivative Instruments Derivative Instruments - Gains (Losses) and Notional Volumes (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU MWh gal	Dec. 31, 2015 USD ($) MMBTU MWh gal	Dec. 31, 2014 USD ($) MMBTU MWh gal
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Gains (losses)	$ (7.6)	$ (9.6)	$ 17.2
Natural gas contracts
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Gains (losses)	$ (12.3)	$ (12.6)	$ 4.0
Notional Disclosures [Abstract]
Notional sales volumes (Dth or MWh) | MMBTU	35.3	24.0	21.4
Petroleum products contracts
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Gains (losses)	$ (2.6)	$ (0.2)	$ 0.5
Notional Disclosures [Abstract]
Notional sales volumes (gallons) | gal	10.3	4.0	9.2
FTRs
Derivative, Gain (Loss) on Derivative, Net [Abstract]
Gains (losses)	$ 7.3	$ 3.2	$ 12.7
Notional Disclosures [Abstract]
Notional sales volumes (Dth or MWh) | MWh	25.3	22.8	26.1