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Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 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 $ (35)   $ (35)     $ (29)    
Assets, fair value disclosure 3,715   3,715     3,945    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 110   110     105    
Derivative liabilities (80)   (80)     (83)    
Cash collateral, net receivable, offset against derivative positions 75   75     76    
Recurring [Member] | Level 1 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 3,011   3,011     3,296    
Derivative liabilities (1)   (1)     (3)    
Recurring [Member] | Level 2 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 629   629     565    
Derivative liabilities (173)   (173)     (175)    
Recurring [Member] | Level 3 [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Assets, fair value disclosure 110   110     113    
Derivative liabilities (16)   (16)     (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 501   501     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 501   501     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 716   716     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 716   716     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 183   183     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 183   183     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 47   47     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 47   47     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 300   300     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 300   300     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 1,622   1,622     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 1,622   1,622     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 (35)   (35)     (29)    
Derivative assets 117   117     118    
Derivative liability, fair value, gross asset and right to reclaim cash, offset 110   110     105    
Derivative liabilities (74)   (74)     (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 (1)   (1)     (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 53   53     42    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (168)   (168)     (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 99   99     104    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (15)   (15)     (10)    
Interest Rate Contract [Member] | Recurring [Member]                
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]                
Derivative assets 36   36     24    
Derivative liabilities (6)   (6)     (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 3   3     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 22   22     15    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (5)   (5)     (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 11   11     9    
Derivative liability, fair value, gross liability including not subject to master netting arrangement (1)   (1)     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 84 $ 86 84 $ 86 $ 83 94 $ 81 $ 60
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings 1 (7) (3) (19)        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) (1) 1 (1) 3        
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net (3) 3 11 5        
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 1 0 2 1        
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements 3 (2) 5 (14)        
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 10 13 10 13 $ 17 $ 9 $ 8 $ 6
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (54) (34) (140) (100)        
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 8 0 6        
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements $ 61 $ 37 $ 139 $ 99