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Kingston Fossil Plant Ash Spill (Details) (USD $)
 6 Months Ended
Mar. 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    
Kingston cost estimate high end of range 1,200,000,000    
Total recorded estimate of Kingston cleanup costs 1,100,000,000    
Period collected in rates   15 years  
Kingston cleanup amounts spent to date 908,000,000    
Remaining estimated Kingston liability 217,000,000    
Kingston insurance proceeds $ 50,000,000