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Variable Interest Entities and Equity Method Investments (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Income (Loss) from Equity Method Investments $ 17.4 $ 18.7 $ 35.9 $ 37.2  
Apple Blossom and Black Oak [Member]          
HLBV Income for Noncontrolling Interests     (4.0)    
Variable Interest Entities (Textuals) [Abstract]          
Noncontrolling Interest 131.0   131.0    
Current Assets [Member] | Apple Blossom and Black Oak [Member]          
Assets [Abstract]          
Assets 8.6   8.6    
Net Property Plant And Equipment [Member] | Apple Blossom and Black Oak [Member]          
Assets [Abstract]          
Assets 235.4   235.4    
Other Noncurrent Assets [Member] | Apple Blossom and Black Oak [Member]          
Assets [Abstract]          
Assets 14.0   14.0    
Total Assets [Member] | Apple Blossom and Black Oak [Member]          
Assets [Abstract]          
Assets 258.0   258.0    
Current Liabilities [Member] | Apple Blossom and Black Oak [Member]          
Liabilities and Equity [Abstract]          
Variable Interest Carrying Amount Liabilities and Equity 6.8   6.8    
Noncurrent Liabilities [Member] | Apple Blossom and Black Oak [Member]          
Liabilities and Equity [Abstract]          
Variable Interest Carrying Amount Liabilities and Equity 4.6   4.6    
Equity [Member] | Apple Blossom and Black Oak [Member]          
Liabilities and Equity [Abstract]          
Variable Interest Carrying Amount Liabilities and Equity 246.6   246.6    
Total Liabilities And Equity [Member] | Apple Blossom and Black Oak [Member]          
Liabilities and Equity [Abstract]          
Variable Interest Carrying Amount Liabilities and Equity $ 258.0   $ 258.0    
JV Wind Farms [Member]          
Equity Method Investment, Ownership Percentage 50.00%   50.00%    
Equity Method Investments $ 403.0   $ 403.0    
Historical Investment Balance 425.0   425.0    
Historical Investment Basis Difference $ (19.0)   (19.0)    
Income (Loss) from Equity Method Investments     (3.0)    
Production Tax Credits Attributable to Equity Method Investment     $ 14.0    
BP Wind Energy [Member]          
Equity Method Investment, Ownership Percentage 50.00%   50.00%    
Berkshire Hathaway Energy [Member]          
Equity Method Investment, Ownership Percentage 50.00%   50.00%    
AEP Transmission Holdco [Member]          
Equity Method Investment, Ownership Percentage 49.50%   49.50%    
AEP Transmission Partner [Member]          
Equity Method Investment, Ownership Percentage 0.50%   0.50%    
ETT [Member]          
Equity Method Investment, Ownership Percentage 50.00%   50.00%    
Equity Method Investments $ 677.0   $ 677.0   $ 666.0
Income (Loss) from Equity Method Investments $ 16.0 $ 15.0 $ 33.0 $ 31.0