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Kingston Fossil Plant Ash Spill Kingston Fossil Plant Ash Spill (Details) (USD $)
 12 Months Ended 30 Months Ended
Sep. 30, 2014
Sep. 30, 2014
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
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 1,100,000,000 1,100,000,000  
Remaining estimated Kingston liability 21,000,000 21,000,000  
Kingston insurance recoveries $ 175,000,000 $ 267,000,000