Derivative Instruments - Gains (Losses) and Notional Volumes (Details) MWh in Millions, MMBTU in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021 USD ($) MMBTU MWh	Dec. 31, 2020 USD ($) MMBTU MWh	Dec. 31, 2019 USD ($) MWh MMBTU
Realized gains (losses) on derivatives
Gains (losses)	$ 154.2	$ (50.0)	$ (10.8)
Natural gas contracts
Realized gains (losses) on derivatives
Gains (losses)	$ 136.5	$ (54.1)	$ (27.1)
Notional sales volumes
Notional sales volumes | MMBTU	197.6	188.6	183.9
FTRs
Realized gains (losses) on derivatives
Gains (losses)	$ 17.7	$ 4.1	$ 16.3
Notional sales volumes
Notional sales volumes | MWh	28.2	29.8	31.2