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, 2016 USD ($) MMBTU MWh gal	Jun. 30, 2015 USD ($) MMBTU MWh gal	Jun. 30, 2016 USD ($) MMBTU MWh gal	Jun. 30, 2015 USD ($) MMBTU MWh gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (19.4)	$ (5.0)	$ (51.0)	$ (10.1)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (20.0)	$ (5.9)	$ (53.5)	$ (13.0)
Notional Volumes
Notional sales volumes (Dth or MWh) | MMBTU	32.7	10.0	82.8	23.3
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ (1.0)	$ 0.1	$ (2.1)	$ 0.0
Notional Volumes
Notional sales volumes (gallons) | gal	3.6	0.8	6.6	1.7
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 1.6	$ 0.8	$ 4.6	$ 2.9
Notional Volumes
Notional sales volumes (Dth or MWh) | MWh	7.4	5.9	15.0	12.1