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Income Taxes: Deferred Income Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2024
Dec. 31, 2023
Deferred Tax Assets, Net [Abstract]    
Regulatory liabilities $ 70.9 $ 74.0
State tax credits 8.4 22.8
Federal NOL 114.9 146.6
State NOL 12.7 16.5
Partnership 11.6 12.2
Credit Carryovers 110.5 110.1
Other deferred tax assets 35.4 33.7
Less: Valuation allowance (1.3) (15.4)
Total deferred tax assets 363.1 400.5
Deferred tax liabilities:    
Accelerated depreciation, amortization and other plant-related differences (729.2) (686.2)
Regulatory assets (49.5) (65.6)
Goodwill (75.9) (67.8)
State deferred tax liability (88.7) (84.5)
Other deferred tax liabilities (44.9) (44.4)
Total deferred tax liabilities (988.2) (948.5)
Net deferred tax liability $ (625.1) $ (548.0)