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Fair Value of Derivative Contracts (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset $ 87 $ 174
Derivative Asset, Contracts Available for Netting (42) (43)
Derivative Liability, Fair Value, Gross Liability (101) (111)
Derivative Liability, Not Offset, Policy Election Deduction 42 43
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]    
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs 0 (15)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Liability Net, Gain (Loss) Included in Earnings 0 (9)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Liability Net, Settlements 0 24
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis with Unobservable Inputs 0 0
Fair Value, Assets Measured on Recurring Basis, Change in Unrealized Gain (Loss) 0 0
Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 205 300
Derivative Asset, Contracts Available for Netting (15) (18)
Derivative Liability, Fair Value, Gross Liability (65) (57)
Derivative Liability, Not Offset, Policy Election Deduction 15 18
Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 166  
Derivative Asset, Contracts Available for Netting (6)  
Derivative Liability, Fair Value, Gross Liability (6) (31)
Derivative Liability, Not Offset, Policy Election Deduction 6 0
Quoted prices in active markets for identical assets (Level 1) [Member] | Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 17 6
Derivative Liability, Fair Value, Gross Liability (3) (29)
Quoted prices in active markets for identical assets (Level 1) [Member] | Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Derivative Liability, Fair Value, Gross Liability 0 0
Quoted prices in active markets for identical assets (Level 1) [Member] | Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0  
Derivative Liability, Fair Value, Gross Liability 0 0
Fair Value, Inputs, Level 2 [Member] | Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 70 168
Derivative Liability, Fair Value, Gross Liability (98) (82)
Fair Value, Inputs, Level 2 [Member] | Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 205 300
Derivative Liability, Fair Value, Gross Liability (65) (57)
Fair Value, Inputs, Level 2 [Member] | Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 166  
Derivative Liability, Fair Value, Gross Liability (6) (31)
Significant unobservable inputs (Level 3) [Member] | Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Derivative Liability, Fair Value, Gross Liability 0 0
Significant unobservable inputs (Level 3) [Member] | Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0 0
Derivative Liability, Fair Value, Gross Liability 0 0
Significant unobservable inputs (Level 3) [Member] | Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Gross Asset 0  
Derivative Liability, Fair Value, Gross Liability 0 0
Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 33 131
Derivative Liability, Fair Value, Amount Not Offset Against Collateral (59) (31)
Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 190 282
Derivative Liability, Fair Value, Amount Not Offset Against Collateral (50) (39)
Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative Asset, Fair Value, Amount Not Offset Against Collateral 160  
Derivative Liability, Fair Value, Amount Not Offset Against Collateral 0 (31)
Contract and Over the Counter [Member] | Energy commodity derivative contracts(a)    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative, Collateral, Obligation to Return Cash, Variation Margin (12) 0
Derivative, Collateral, Right to Reclaim Cash, Variation Margins 0 37
Contract and Over the Counter [Member] | Interest rate swap agreements    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative, Collateral, Obligation to Return Cash, Variation Margin 0 0
Derivative, Collateral, Right to Reclaim Cash, Variation Margins 0 0
Contract and Over the Counter [Member] | Currency Swap [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative, Collateral, Obligation to Return Cash, Variation Margin 0  
Derivative, Collateral, Right to Reclaim Cash, Variation Margins $ 0 $ 0