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Commitments and Contingencies Commitments and Contingencies - Legal Proceedings (Details)
 1 Months Ended 12 Months Ended
Jul. 31, 2013
Legal_actions
May. 31, 2013
Dec. 31, 2011
Legal_actions
Apr. 30, 2011
Groups
Agreements
Mar. 31, 2010
USD ($)
People
Jun. 30, 2008
Legal_actions
Sep. 30, 2015
USD ($)
Legal_actions
Dec. 31, 2010
Years
Apr. 30, 2015
Legal_actions
Apr. 14, 2015
Megawatts
Sep. 30, 2014
USD ($)
Jun. 30, 2011
USD ($)
Units
Megawatts
Jun. 30, 2010
USD ($)
Legal Proceedings                          
Accrual for Environmental Loss Contingencies, Gross             $ 23,000,000       $ 15,000,000    
Megawatts option 1 | Megawatts                   700      
TDEC civil penalties related to Gallatin             $ 17,000            
Number of units at John Sevier Fossil             4            
Units Affected by Paradise Case             2            
General                          
Legal Proceedings                          
Legal loss contingency accrual             $ 115,000,000            
Environmental Agreements                          
Legal Proceedings                          
Number of similar environmental agreements entered into | Agreements       2                  
Number of environmental agreements entered into with the EPA | Agreements       1                  
Number of environmental agreements entered into with Alabama, Kentucky, North Carolina, and Tennessee | Agreements       1                  
Number of environmental agreements entered into with environmental advocacy groups | Groups       3                  
Number of units to be idled | Units                       18  
Megawatts option 1 | Megawatts                       2,200  
Megawatts option 2 | Megawatts                       3,500  
Amount to be invested in certain environmental projects                       $ 290,000,000  
Amount to be provided to fund environmental projects                       60,000,000  
Amount to pay civil penalties                       $ 10,000,000  
Kingston Ash Spill                          
Legal Proceedings                          
Number of lawsuits filed | Legal_actions             78            
Number of lawsuits dismissed | Legal_actions             15   2        
Loss Contingency, Claims Dismissed, Number | Legal_actions             63            
Number of active lawsuits | Legal_actions             9            
Payments for Legal Settlements             $ 28,000,000            
Loss Contingency, Claims Settled, Number | Legal_actions             7            
Civil penalty order issued June 1, 2010                         $ 12,000,000
Amount of civil penalty order satisfied             $ 10,000,000            
Credit against civil penalty order             2,000,000            
Down payment on natural resource damages             750,000            
Case Involving Tennessee Valley Authority Retirement System                          
Legal Proceedings                          
Number of participants that filed suit | People         8                
Contribution related to TVARS case         $ 1,000,000,000                
Retirement age of eligibility for cost of living adjustment before January 1, 2010 | Years               55          
Retirement age of eligibility for cost of living adjustment after January 1, 2010 | Years               60          
Administrative Proceedings Regarding Sequoyah U1 and U2 [Member]                          
Legal Proceedings                          
Number of contentions submitted by BREDL BEST and MATRR   8                      
Opposed contentions   8                      
Number of petitioners with standing 1                        
Number of petitioners 3                        
Number of contentions dismissed | Legal_actions 7                        
Number of contentions left | Legal_actions 1                        
Administrative Proceedings Regarding Bellefonte Units 3 and 4                          
Legal Proceedings                          
Number of contentions left | Legal_actions           2              
Number of admitted contentions submitted by BREDL and SACE | Legal_actions           4              
Number of contentions submitted by BREDL and SACE | Legal_actions           20              
Number of contentions no longer admitted | Legal_actions           2              
Petitions Resulting from Japanese Nuclear Events                          
Legal Proceedings                          
Number of requests accepted by the NRC | Legal_actions     5                    
Other long-term liabilities | General                          
Legal Proceedings                          
Legal loss contingency accrual             55,000,000            
Accounts payable and accrued liabilities | General                          
Legal Proceedings                          
Legal loss contingency accrual             $ 60,000,000