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Accounting for Derivative Instruments and Hedging Activities - Pre-tax effects of economic hedges not designated a cash flow hedges (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Unrealized mark-to-market results
Reversal of previously recognized unrealized losses on settled positions related to economic hedges	$ 19	$ 1
Reversal of acquired gain positions related to economic hedges	(2)	0
Net unrealized gains on open positions related to economic hedges	3	205
Total unrealized mark-to-market gains for economic hedging activities	20	206
Reversal of previously recognized unrealized gains on settled positions related to trading activity	(6)	(3)
Net unrealized gains on open positions related to trading activity	13	11
Total unrealized mark-to-market gains for trading activity	7	8
Total unrealized gains	27	214
Credit Risk Related Contingent Features
Unrealized gain (loss) from open economic hedge positions	3	205
Collateral due on net liability position that has not been called by a certain marginable agreement counterparty	3
Adequate Assurance Clauses
Credit Risk Related Contingent Features
Derivative net liability position, collateral required Contracts with Credit Rating Contingent Feature	17
Credit Rating Contingent Features
Credit Risk Related Contingent Features
Derivative net liability position, collateral required Contracts with Credit Rating Contingent Feature	23
Commodity contracts
Unrealized mark-to-market results
Total unrealized gains	27	214
Commodity contracts | Operating revenues
Unrealized mark-to-market results
Total unrealized gains	27	(88)
Commodity contracts | Cost of operations
Unrealized mark-to-market results
Total unrealized gains	0	302
Interest rate contracts
Unrealized mark-to-market results
Total unrealized gains	$ (9)	$ 12

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Unrealized Gain (Loss) on Derivatives, Mark To Market, Economic Hedging Activities

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The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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Amount of realized and unrealized gain (loss) of derivative instruments not designated or qualifying as hedging instruments.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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Amount of unrealized gain (loss) related to derivatives designated as cash flow hedging instruments, recorded in results of operations to the extent that the cash flow hedge is determined to be ineffective.

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