Derivative Instruments - Derivative Assets and Liabilities (Details) $ in Millions	Dec. 31, 2023 USD ($) Instruments	Dec. 31, 2022 USD ($) Instruments
Derivative assets
Current derivative assets	$ 17.9	$ 59.2
Long-term derivative assets	0.1	15.6
Total derivative assets	$ 18.0	$ 74.8
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other	Other
Derivative liabilities
Current derivative liabilities	$ 89.0	$ 88.2
Long-term derivative Iiabilities	17.4	8.4
Total derivative liabilities	$ 106.4	$ 96.6
Current derivative liabilities balance sheet location	Other	Other
Long-term derivative liabilities balance sheet location	Other	Other
Natural gas contracts
Derivative assets
Current derivative assets	$ 10.4	$ 32.5
Long-term derivative assets	0.1	0.0
Derivative liabilities
Current derivative liabilities	78.1	88.2
Long-term derivative Iiabilities	8.0	8.4
FTRs and TCRs
Derivative assets
Current derivative assets	7.2	7.8
Derivative liabilities
Current derivative liabilities	0.0	0.0
Coal contracts
Derivative assets
Current derivative assets	0.3	18.9
Long-term derivative assets	0.0	15.6
Derivative liabilities
Current derivative liabilities	10.9	0.0
Long-term derivative Iiabilities	$ 9.4	$ 0.0
Hedging instruments
Derivative instruments
Number of derivatives designated as hedging instruments | Instruments	0	0