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Fair Value Measurements (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Dec. 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 $ (27)   $ (29)  
Assets, fair value disclosure 3,442   3,945  
Derivative liability, fair value, gross asset and right to reclaim cash, offset 111   105  
Derivative liabilities (105)   (83)  
Cash collateral, net receivable, offset against derivative positions 84   76  
Recurring [Member] | Level 1 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Assets, fair value disclosure 2,820   3,296  
Derivative liabilities 0   (3)  
Recurring [Member] | Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Assets, fair value disclosure 535   565  
Derivative liabilities (199)   (175)  
Recurring [Member] | Level 3 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Assets, fair value disclosure 114   113  
Derivative liabilities (17)   (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 440   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  
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 440   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  
Money market mutual funds [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 413   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 413   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  
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  
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   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   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  
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  
International government obligations [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 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  
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   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  
Corporate obligations [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 37   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  
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 37   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  
Municipal obligations [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 1   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  
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 1   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  
United States companies [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 278   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 278   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  
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  
International companies [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 1,760   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 1,760   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  
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  
Investment funds [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Available-for-sale securities 185   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 185   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  
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  
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 (27)   (29)  
Derivative assets 108   118  
Derivative liability, fair value, gross asset and right to reclaim cash, offset 111   105  
Derivative liabilities (97)   (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 1   1  
Derivative liability, fair value, gross liability including not subject to master netting arrangement 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 36   42  
Derivative liability, fair value, gross liability including not subject to master netting arrangement (191)   (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 98   104  
Derivative liability, fair value, gross liability including not subject to master netting arrangement (17)   (10)  
Interest Rate Contract [Member] | Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Derivative assets 33   24  
Derivative liabilities (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 0   0  
Derivative liability, fair value, gross liability including not subject to master netting arrangement 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   15  
Derivative liability, fair value, gross liability including not subject to master netting arrangement (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 16   9  
Derivative liability, fair value, gross liability including not subject to master netting arrangement 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 81 $ 72 94 $ 60
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings 0 (12)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) 1 2    
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 9 (1)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases 1 0    
Fair Value, Measurements With Unobservable Inputs Reconciliation, Recurring Basis, Assets and Liability, Net, Settlements 4 (1)    
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 16 9 $ 9 $ 6
Fair Value, Assets and Liabilities Measured on Recurring Basis, Gain (Loss) Included in Earnings (30) (27)    
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included in Other Comprehensive Income (Loss) 0 0    
Fair Value, Measurements with Unobservable Inputs Reconciliation, Recurring Basis, Gain (Loss) Included In Regulatory Assets and Liabilities, Net 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 $ 23 $ 22