Derivative Instruments - Derivative Assets and Liabilities (Details) $ in Millions	Dec. 31, 2021 USD ($) Instruments	Dec. 31, 2020 USD ($)
Derivative assets
Other current derivative assets	$ 107.0	$ 17.0
Other long-term derivative assets	13.0	0.9
Derivative assets	120.0	17.9
Derivative liabilities
Other current derivative liabilities	14.0	20.5
Other long-term derivative Iiabilities	1.1	1.6
Derivative liabilities	$ 15.1	22.1
Designated as Hedging Instrument
Derivative instruments
Derivative, Number of Instruments Held | Instruments	0
Natural gas contracts
Derivative assets
Other current derivative assets	$ 60.6	13.0
Other long-term derivative assets	4.0	0.7
Derivative liabilities
Other current derivative liabilities	14.0	12.9
Other long-term derivative Iiabilities	1.1	1.2
FTRs
Derivative assets
Other current derivative assets	2.4	2.4
Derivative liabilities
Other current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Other current derivative assets	44.0	1.6
Other long-term derivative assets	9.0	0.2
Derivative liabilities
Other current derivative liabilities	0.0	0.8
Other long-term derivative Iiabilities	0.0	0.4
Interest rate swaps
Derivative assets
Other current derivative assets	0.0	0.0
Derivative liabilities
Other current derivative liabilities	$ 0.0	$ 6.8