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Kingston Fossil Plant Ash Spill (Details)
Cubic_yards in Millions, $ in Millions
 9 Months Ended 36 Months Ended
Jun. 30, 2015
USD ($)
Mar. 31, 2015
USD ($)
Dec. 31, 2008
Dredge_cells
Cubic_yards
Kingston Fossil Plant Ash Spill      
Number of dredge cells that failed | Dredge_cells     1
Cubic yards of water and coal fly ash that flowed out of the cell | Cubic_yards     5
Total recorded estimate of Kingston cleanup costs $ 1,100    
Period collected in rates 15 years    
Kingston cleanup amounts spent to date $ 1,100    
Remaining estimated Kingston liability 18    
Assets held for sale $ 12    
Kingston insurance recoveries   $ 317