Derivative Instruments - Gains (Losses) and Notional Volumes (Details) gal in Millions, MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) MWh MMBTU gal	Dec. 31, 2019 USD ($) MWh MMBTU gal	Dec. 31, 2018 USD ($) MMBTU MWh gal
Realized gains (losses) on derivatives
Gains (losses)	$ (50.0)	$ (10.8)	$ 42.1
Natural gas contracts
Realized gains (losses) on derivatives
Gains (losses)	$ (54.1)	$ (27.1)	$ 24.6
Notional sales volumes
Notional sales volumes | MMBTU	188.6	183.9	173.2
Petroleum products contracts
Realized gains (losses) on derivatives
Gains (losses)	$ 0.0	$ 0.0	$ 1.6
Notional sales volumes
Notional sales volumes (gallons) | gal	0.0	0.0	6.0
FTRs
Realized gains (losses) on derivatives
Gains (losses)	$ 4.1	$ 16.3	$ 15.9
Notional sales volumes
Notional sales volumes | MWh	29.8	31.2	30.5