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, 2016 USD ($) MMBTU MWh gal	Sep. 30, 2015 USD ($) MMBTU MWh gal	Sep. 30, 2016 USD ($) MMBTU MWh gal	Sep. 30, 2015 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 3.3	$ (11.0)	$ (47.7)	$ (21.1)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (3.4)	$ (13.2)	$ (56.9)	$ (26.2)
Notional Volumes
Notional sales volumes (Dth or MWh) | MMBTU	30.5	24.2	113.3	47.5
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (0.4)	$ (0.9)	$ (2.5)	$ (0.9)
Notional Volumes
Notional sales volumes (gallons) | gal	4.3	2.8	10.9	4.5
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 7.1	$ 3.1	$ 11.7	$ 6.0
Notional Volumes
Notional sales volumes (Dth or MWh) | MWh	9.9	8.6	24.9	20.7