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Kingston Fossil Plant Ash Spill (Details) (USD $)
 3 Months Ended 21 Months Ended
Dec. 31, 2013
Dec. 31, 2013
Oct. 02, 2009
Dec. 31, 2008
Dredge_cells
Cubic_yards
Kingston Fossil Plant Ash Spill        
Number of dredge cells that failed       1
Cubic yards of water and coal fly ash that flowed out of the cell       5,000,000
Kingston cost estimate low end of range $ 1,100,000,000 $ 1,100,000,000    
Kingston cost estimate high end of range 1,200,000,000 1,200,000,000    
Total recorded estimate of Kingston cleanup costs 1,100,000,000 1,100,000,000    
Period collected in rates     15 years  
Kingston cleanup amounts spent to date 978,000,000 978,000,000    
Remaining estimated Kingston liability 147,000,000 147,000,000    
Proceeds from Insurance Settlement, Operating Activities 150,000,000 242,000,000    
Interest Income from Insurance Proceeds $ 11,000,000