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 ($) MWh MMBTU gal	Dec. 31, 2015 USD ($) MWh MMBTU gal	Dec. 31, 2014 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (49.5)	$ (45.7)	$ 20.5
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (59.6)	$ (50.5)	$ 7.3
Notional Volumes
Notional sales volumes (Dth or MWh) | MMBTU	151.1	86.2	40.5
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (3.2)	$ (1.9)	$ 0.5
Notional Volumes
Notional sales volumes (gallons) | gal	14.7	7.8	9.2
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 13.3	$ 6.7	$ 12.7
Notional Volumes
Notional sales volumes (Dth or MWh) | MWh	33.7	27.3	26.1