Fair Value of Derivative Contracts (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	$ 551	$ 458
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(171)	(172)
Energy Related Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	221	87
Derivative Asset, Contracts Available for Netting	(39)	(42)
Derivative, Collateral, Obligation to Return Cash, Variation Margin	(25)	(12)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	157	33
Derivative Liability, Fair Value, Gross Liability	(50)	(101)
Derivative Liability, Not Offset, Policy Election Deduction	39	42
Derivative, Collateral, Right to Reclaim Cash, Variation Margins	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(11)	(59)
Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	133	205
Derivative Asset, Contracts Available for Netting	(7)	(15)
Derivative, Collateral, Obligation to Return Cash, Variation Margin	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	126	190
Derivative Liability, Fair Value, Gross Liability	(115)	(65)
Derivative Liability, Not Offset, Policy Election Deduction	7	15
Derivative, Collateral, Right to Reclaim Cash, Variation Margins	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	(108)	(50)
Currency Swap [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	197	166
Derivative Asset, Contracts Available for Netting	(6)	(6)
Derivative, Collateral, Obligation to Return Cash, Variation Margin	0	0
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	191	160
Derivative Liability, Fair Value, Gross Liability	(6)	(6)
Derivative Liability, Not Offset, Policy Election Deduction	6	6
Derivative, Collateral, Right to Reclaim Cash, Variation Margins	0	0
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	0	0
Quoted prices in active markets for identical assets (Level 1) [Member] \| Energy Related Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	28	17
Derivative Liability, Fair Value, Gross Liability	(11)	(3)
Quoted prices in active markets for identical assets (Level 1) [Member] \| Interest rate contract
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	0
Derivative Liability, Fair Value, Gross Liability	0	0
Fair Value, Inputs, Level 2 [Member] \| Energy Related Derivative
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	193	70
Derivative Liability, Fair Value, Gross Liability	(39)	(98)
Fair Value, Inputs, Level 2 [Member] \| Interest rate contract
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	133	205
Derivative Liability, Fair Value, Gross Liability	(115)	(65)
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	197	166
Derivative Liability, Fair Value, Gross Liability	(6)	(6)
Significant unobservable inputs (Level 3) [Member] \| Energy Related Derivative
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 contract
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	0
Derivative Liability, Fair Value, Gross Liability	$ 0	$ 0

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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