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Income Tax Expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Current Income Tax Expense [Abstract]      
Federal $ 9.4 $ 1.2 $ 0.0
State 0.9 6.1 0.0
Total Current Income Tax Expense 10.3 7.3 0.0
Deferred Income Tax Expense (Benefit) [Abstract]      
Federal (6.0) (32.8) (37.2)
State 24.0 (5.2) 10.8
Investment Tax Credit Amortization (0.4) (0.5) (0.5)
Total Deferred Income Tax Expense (Benefit) 17.6 (38.5) (26.9)
Total Income Tax Expense (Benefit) $ 27.9 $ (31.2) $ (26.9)