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Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Cash collateral, net receivable, offset against derivative positions $ 84   $ 84     $ 74    
Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (19)   (19)     (22)    
Assets, fair value disclosure 3,341   3,341     2,878    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 103   103     96    
Derivative liabilities (139)   (139)     (145)    
Cash collateral, net receivable, offset against derivative positions 84   84     74    
Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 2,769   2,769     2,339    
Derivative liabilities (2)   (2)     (3)    
Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 485   485     467    
Derivative liabilities (223)   (223)     (210)    
Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 106   106     94    
Derivative liabilities (17)   (17)     (28)    
Mortgage Loans on Real Estate [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 408   408     359    
Mortgage Loans on Real Estate [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Mortgage Loans on Real Estate [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 408   408     359    
Mortgage Loans on Real Estate [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Money market mutual funds [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 773   773     586    
Money market mutual funds [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 773   773     586    
Money market mutual funds [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Money market mutual funds [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
United States government obligations [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 161   161     161    
United States government obligations [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 161   161     161    
United States government obligations [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
United States government obligations [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
International government obligations [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 4   4     3    
International government obligations [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
International government obligations [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 4   4     3    
International government obligations [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Corporate obligations [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 36   36     36    
Corporate obligations [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Corporate obligations [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 36   36     36    
Corporate obligations [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Municipal obligations [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 2   2     2    
Municipal obligations [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Municipal obligations [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 2   2     2    
Municipal obligations [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Agency, asset and mortgage-backed obligations [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 1   1     2    
Agency, asset and mortgage-backed obligations [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Agency, asset and mortgage-backed obligations [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 1   1     2    
Agency, asset and mortgage-backed obligations [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
United States companies [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 270   270     250    
United States companies [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 270   270     250    
United States companies [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
United States companies [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
International companies [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 1,388   1,388     1,190    
International companies [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 1,388   1,388     1,190    
International companies [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
International companies [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Investment funds [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 175   175     147    
Investment funds [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 175   175     147    
Investment funds [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Investment funds [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 0   0     0    
Commodity derivative [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative Asset, Fair Value, Gross Liability and Obligation to Return Cash, Offset (19)   (19)     (22)    
Derivative assets 105   105     119    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 103   103     96    
Derivative liabilities (125)   (125)     (132)    
Commodity derivative [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 2   2     5    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (2)   (2)     (2)    
Commodity derivative [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 26   26     49    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (211)   (211)     (199)    
Commodity derivative [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 96   96     87    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (15)   (15)     (27)    
Interest Rate Contract [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative assets 18   18     23    
Derivative liabilities (14)   (14)     (13)    
Interest Rate Contract [Member] | Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 0   0     0    
Derivative liability, fair value, gross liability including not subject to master netting arrangement 0   0     (1)    
Interest Rate Contract [Member] | Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 8   8     16    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (12)   (12)     (11)    
Interest Rate Contract [Member] | Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative asset, fair value, gross asset including not subject to master netting arrangement 10   10     7    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (2)   (2)     (1)    
Commodity derivative [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair Value Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability), Net, Value 81 $ 44 81 $ 44 $ 72 60 $ 58 $ 47
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings 0 20 (12) 1        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 (6) 2 0        
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 3 5 2 11        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 1   1          
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements (11) (5) (12) (9)        
Interest Rate Lock Commitments [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair Value Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability), Net, Value 8 14 8 14 9 6 11 4
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (39) (29) (66) (54)        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 0 0 0        
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 0 0 0 0        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0   (2)          
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements 40 0 62 44        
Auction rate securities [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Fair Value Measurement With Unobservable Inputs Reconciliation, Recurring Basis, Asset (Liability), Net, Value 0 18 0 18 $ 0 $ 0 $ 26 $ 44
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings 0 0 0 0        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 (2) 0 (6)        
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 0 0 0 0        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0   0          
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements $ 0 $ 26 $ 0 $ 0