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Income Taxes - Textual (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Valuation Allowance [Line Items]          
Tax Expense or Benefit Resulting from the Tax Cuts and Jobs Act $ 1.2        
Bonus Depreciation Deductions, Tax Cuts and Jobs Act 8.8        
Tax expense at statutory rate (percent)   35.00% 35.00% 35.00%  
Income tax benefit allocated to Capital in excess of par value   $ (25.1) $ (4.2) $ (11.7)  
Estimated realization of additional tax benefits   51.3 0.1 0.3  
Unrecognized Tax Benefits, Increase Resulting from Current Period Tax Positions, Pre-Payment Claims Denials       39.0  
Changes to valuation allowance during the period   7.9 0.3 4.6  
Remaining valuation allowance 35.8 35.8 27.9    
Unrecognized income tax benefits 0.3 0.3 2.8 2.9 $ 0.9
Unrecognized tax benefits, decrease   2.5      
Unrecognized tax benefits that would have affected effective tax rate if recognized 0.3 0.3 $ 2.8 $ 2.9  
Federal [Member]          
Valuation Allowance [Line Items]          
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability 10.1        
State [Member]          
Valuation Allowance [Line Items]          
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability 14.7        
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount, Tax Cuts and Jobs Act   5.8      
Net operating loss $ 77.3 $ 77.3