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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 0 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended 3 Months Ended 9 Months Ended
Apr. 16, 2012
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Private Partnerships
Jun. 30, 2012
Private Partnerships
Jun. 30, 2013
Private Partnerships
Jun. 30, 2012
Private Partnerships
Jun. 30, 2013
Commodity Contract Derivatives
Jun. 30, 2012
Commodity Contract Derivatives
Jun. 30, 2013
Commodity Contract Derivatives
Jun. 30, 2012
Commodity Contract Derivatives
Jun. 30, 2013
Interest Rate Swaption
Jun. 30, 2012
Interest Rate Swaption
Jun. 30, 2013
Interest Rate Swaption
Jun. 30, 2012
Interest Rate Swaption
Jun. 30, 2013
Minimum
tons-per-year
Jun. 30, 2013
Maximum
tons-per-year
Jun. 30, 2013
Assets
Jun. 30, 2013
Liabilities, Total
Fair Value Measurements                                      
Balances at beginning of period       $ 137,000,000 $ 36,000,000 $ 53,000,000 $ 22,000,000 $ (148,000,000) $ (311,000,000) $ (267,000,000) $ 239,000,000 $ 0 $ (993,000,000) $ 0 $ (1,077,000,000)        
Purchases       9,000,000 9,000,000 93,000,000 21,000,000 0 0 0 0 0 0 0 0        
Issuances       0 0 0 0 0 0 0 0 0 0 0 0        
Sales       (1,000,000) 0 (3,000,000) (2,000,000) 0 0 0 0 0 0 0 0        
Settlements       0 0 0 0 0 0 0 0 0 993,000,000 0 1,077,000,000        
Net realized (gains) losses recognized in income   0                                  
Net unrealized gains (losses) deferred as regulatory assets and liabilities       5,000,000 0 7,000,000 4,000,000 39,000,000 (20,000,000) 158,000,000 (570,000,000) 0 0 0 0        
Balances at end of period       150,000,000 45,000,000 150,000,000 45,000,000 (109,000,000) (331,000,000) (109,000,000) (331,000,000) 0 0 0 0        
Proceeds from derivative instrument financing (60,000,000) 0 (60,000,000)                                
Assets                                      
Commodity contract derivatives, fair value disclosure                                   8,000,000 117,000,000
Fair value inputs, counterparty credit risk                               21.00% [1] 21.00% [1]    
Fair value measurements tons per year                               900,000,000 1,000,000,000    
Price per ton                               14.45 90.31    
Liabilities                                      
Commodity contract derivatives, fair value disclosure                                   8,000,000 117,000,000
Fair value measurements tons per year                               900,000,000 1,000,000,000    
Price per ton                               $ 14.45 $ 90.31    
[1] Applies to only one contract.