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Income Taxes (Narrative) (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Taxes [Line Items]        
Tax expense at statutory federal income tax rate, rate   21.00% 35.00% 35.00%
Effective income tax rate   78.10% 71.00% 35.50%
Increase (Decrease) in Effective Tax Rate   7.10% 35.50%  
State regulatory proceedings, value   $ 127.8 $ 0.0 $ 0.0
Increase in State Income Taxes   7.1    
Tax Benefit From Excess Deferred Tax Amortization Related To Regulatory LIabilities   26.9    
Expected benefits to be realized $ 65.8 $ 88.5 $ 65.8  
Income Tax Rate Change Period     6 years  
Operating Loss Carryforwards, Limitations on Use   80.00%    
Operating Loss Carryforwards, Valuation Allowance   $ 759.6    
Operating Loss Carryforwards, Valuation Allowance, years prior to TCJA   508.5    
Deferred Tax Assets, Tax Credit Carryforwards, Alternative Minimum Tax   1.7    
Unrecognized Tax Benefits, Income Tax Penalties Accrued 0.0 0.0 $ 0.0 0.0
Reduction in Deferred Tax Liability Due To Tax Law Change     1,300.0  
Reduction in Regulatory Deferred tax Liabilities     400.0  
Increase in Regulatory Liability Due to Tax Law Change     1,500.0  
Increase in Tax Expense Due to Tax Law Change     200.0  
Remeasurement due to TCJA, value $ 161.1 $ 0.0 $ 161.1 $ 0.0
Minimum        
Income Taxes [Line Items]        
Tax Credit Carryforward, Expiration Date   Dec. 31, 2028    
Maximum        
Income Taxes [Line Items]        
Tax Credit Carryforward, Expiration Date   Dec. 31, 2037    
Federal tax on excess stock compensation [Domain]        
Income Taxes [Line Items]        
Change in tax expense   $ 120.7