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Fair Value Measurements Fair Value Measurements Using Significant Unobservable Inputs (Details) (USD $)
 0 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended 3 Months Ended 6 Months Ended
Apr. 16, 2013
Mar. 31, 2013
Mar. 31, 2013
Private Partnerships
Mar. 31, 2012
Private Partnerships
Mar. 31, 2013
Private Partnerships
Mar. 31, 2012
Private Partnerships
Mar. 31, 2013
Commodity Contract Derivatives
Mar. 31, 2012
Commodity Contract Derivatives
Mar. 31, 2013
Commodity Contract Derivatives
Mar. 31, 2012
Commodity Contract Derivatives
Mar. 31, 2013
Interest Rate Swaption
Mar. 31, 2012
Interest Rate Swaption
Mar. 31, 2013
Interest Rate Swaption
Mar. 31, 2012
Interest Rate Swaption
Mar. 31, 2013
Minimum
tons-per-year
Mar. 31, 2013
Maximum
tons-per-year
Mar. 31, 2013
Assets
Mar. 31, 2013
Liabilities
Fair Value Measurements                                    
Balances at beginning of period     $ 66,000,000 $ 28,000,000 $ 53,000,000 $ 22,000,000 $ (224,000,000) $ 4,000,000 $ (267,000,000) $ 239,000,000 $ 0 $ (1,128,000,000) $ 0 $ (1,077,000,000)        
Purchases     70,000,000 7,000,000 83,000,000 13,000,000 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 (2,000,000) (1,000,000) 0 0 0 0 0 0 0 0        
Settlements     0 0 0 0 0 0 0 0 0 0 0 0        
Net realized (gains) losses recognized in income   0                                
Net unrealized gains (losses) deferred as regulatory assets and liabilities     2,000,000 1,000,000 3,000,000 2,000,000 76,000,000 (315,000,000) 119,000,000 (550,000,000) 0 135,000,000 0 84,000,000        
Balances at end of period     137,000,000 36,000,000 137,000,000 36,000,000 (148,000,000) (311,000,000) (148,000,000) (311,000,000) 0 (993,000,000) 0 (993,000,000)        
Proceeds from derivative instrument financing (60,000,000)                                  
Assets                                    
Commodity contract derivatives, fair value disclosure                                 7,000,000 155,000,000
Fair value inputs, counterparty credit risk                             9.40% [1] 9.40% [1]    
Fair value measurements tons per year                             1,000,000,000 1,100,000,000    
Price per ton                             13.75 86.83    
Liabilities                                    
Commodity contract derivatives, fair value disclosure                                 7,000,000 155,000,000
Fair value measurements tons per year                             1,000,000,000 1,100,000,000    
Price per ton                             $ 13.75 $ 86.83    
[1] Applies to only one contract.