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Kingston Fossil Plant Ash Spill (Details) (USD $)
 3 Months Ended 9 Months Ended 27 Months Ended
Jun. 30, 2014
Jun. 30, 2014
Jun. 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
Kingston cost estimate low end of range $ 1,100,000,000 $ 1,100,000,000 $ 1,100,000,000  
Kingston cost estimate high end of range 1,200,000,000 1,200,000,000 1,200,000,000  
Total recorded estimate of Kingston cleanup costs 1,100,000,000 1,100,000,000 1,100,000,000  
Period collected in rates   15 years    
Kingston cleanup amounts spent to date 1,000,000,000 1,000,000,000 1,000,000,000  
Remaining estimated Kingston liability 82,000,000 82,000,000 82,000,000  
Kingston insurance recoveries $ 14,000,000 $ 175,000,000 $ 267,000,000