Derivative Instruments - Gains (Losses) and Notional Volumes (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) MWh MMBTU gal	Dec. 31, 2017 USD ($) MWh MMBTU gal	Dec. 31, 2016 USD ($) MWh MMBTU gal
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 42.1	$ 4.7	$ (49.5)
Natural gas contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 24.6	$ (8.0)	$ (59.6)
Notional Sales Volumes
Notional sales volumes (Dth or MWh) | MMBTU	173.2	123.1	151.1
Petroleum products contracts
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 1.6	$ (1.3)	$ (3.2)
Notional Sales Volumes
Notional sales volumes (gallons) | gal	6.0	18.0	14.7
FTRs
Realized Gain (Loss) on Derivatives, Net
Gains (Losses)	$ 15.9	$ 14.0	$ 13.3
Notional Sales Volumes
Notional sales volumes (Dth or MWh) | MWh	30.5	36.2	33.7