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Income Taxes (Schedule Of Reasons Behind Differences In Computation Of Total Income Taxes) (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Schedule of Reasons Behind Differences in Computation of Total Income Taxes [Line Items]        
Book income (loss) before income taxes   $ (230.6) $ 443.0 $ 513.6
Tax expense (benefit) at statutory federal income tax rate, value   $ (48.4) $ 155.0 $ 179.8
Tax expense (benefit) at statutory federal income tax rate, rate   21.00% 35.00% 35.00%
Increases (reductions) in taxes resulting from:        
State income taxes, net of federal income tax benefit, value   $ 24.7 $ 6.9 $ 11.3
State income taxes, net of federal income tax benefit, rate   (10.70%) 1.50% 2.20%
Regulatory treatment of depreciation differences, value   $ (29.3) $ (2.4) $ (1.5)
Regulatory treatment of depreciation differences, rate   12.70% (0.50%) (0.30%)
Charitable contribution carryover, value   $ 0.0 $ (1.2) $ (2.8)
Charitable contribution carryover, rate   (0.00%) (0.30%) (0.50%)
State regulatory proceedings, value   $ (127.8) $ 0.0 $ 0.0
State regulatory proceedings, rate   55.40% 0.00% 0.00%
Remeasurement due to TCJA, value $ 161.1 $ 0.0 $ 161.1 $ 0.0
Remeasurement due to TCJA, rate   0.00% 36.40% 0.00%
Employee stock ownership plan dividends and other compensation, value   $ (2.2) $ (6.5) $ (9.5)
Employee stock ownership plan dividends and other compensation, rate   1.00% (1.50%) (1.80%)
Other adjustments, value   $ 3.0 $ 1.6 $ (0.8)
Other adjustments, rate   (1.30%) 0.40% (0.10%)
Income Taxes   $ (180.0) $ 314.5 $ 182.1
Income Taxes, rate   78.10% 71.00% 35.50%