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Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
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 $ (33)   $ (33)     $ (29)    
Assets, fair value disclosure 3,584   3,584     3,945    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 117   117     105    
Derivative liabilities (93)   (93)     (83)    
Cash collateral, net receivable, offset against derivative positions 84   84     76    
Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 2,631   2,631     3,296    
Derivative liabilities 0   0     (3)    
Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 868   868     565    
Derivative liabilities (192)   (192)     (175)    
Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 118   118     113    
Derivative liabilities (18)   (18)     (10)    
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 763   763     465    
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 763   763     465    
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 600   600     685    
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 600   600     685    
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 184   184     176    
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 184   184     176    
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     5    
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     5    
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    
United States companies [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Available-for-sale securities 289   289     288    
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 289   289     288    
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 1,370   1,370     1,968    
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 1,370   1,370     1,968    
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 187   187     178    
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 187   187     178    
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 (33)   (33)     (29)    
Derivative assets 114   114     118    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 117   117     105    
Derivative liabilities (85)   (85)     (75)    
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 0   0     1    
Derivative liability, fair value, gross liability including not subject to master netting arrangement 0   0     (3)    
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 46   46     42    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (184)   (184)     (167)    
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 101   101     104    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (18)   (18)     (10)    
Interest Rate Contract [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative assets 35   35     24    
Derivative liabilities (8)   (8)     (8)    
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 1   1     0    
Derivative liability, fair value, gross liability including not subject to master netting arrangement 0   0     0    
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 17   17     15    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (8)   (8)     (8)    
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 17   17     9    
Derivative liability, fair value, gross liability including not subject to master netting arrangement 0   0     0    
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 83 $ 81 83 $ 81 $ 81 94 $ 72 $ 60
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (4) 0 (4) (12)        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) (1) 0 0 2        
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 5 3 14 2        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 0 1 1 1        
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements (2) (11) 2 (12)        
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 17 8 17 8 $ 16 $ 9 $ 9 $ 6
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (56) (39) (86) (66)        
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 0 0 (2)        
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements $ 55 $ 40 $ 78 $ 62