Accounting for Derivative Instruments and Hedging Activities - Pre-tax Effects of Economic Hedges not Designated as Cash Flow Hedges (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrealized Mark To Market Results [Abstract]
Reversal of previously recognized unrealized loss/(gains) on settled positions related to economic hedges	$ 47	$ (128)	$ (162)
Reversal of acquired gain positions related to economic hedges	0	(12)	(22)
Net unrealized gains/(losses) on open positions related to economic hedges	146	6	(9)
Total unrealized mark-to-market gains/(losses) for economic hedging activities	193	(134)	(193)
Reversal of previously recognized unrealized (gains)/losses on settled positions related to trading activity	(25)	10	(46)
Reversal of acquired gain positions related to trading activity	0	0	(14)
Net unrealized gains/(losses) on open positions related to trading activity	14	18	(16)
Total unrealized mark-to-market (losses)/gains for trading activity	(11)	28	(76)
Total unrealized gains/(losses)	182	(106)	(269)
Credit Risk Related Contingent Features [Abstract]
Collateral required contracts with credit rating contingent features in a net liability position	4
Adequate Assurance Clauses
Credit Risk Related Contingent Features [Abstract]
Collateral required for contracts in net liability positions that have adequate assurance clauses	25
Credit Rating Contingent Features
Credit Risk Related Contingent Features [Abstract]
Collateral required for contracts in net liability positions that have adequate assurance clauses	7
Commodity contracts
Unrealized Mark To Market Results [Abstract]
Total unrealized gains/(losses)	182	(106)	(269)
Commodity contracts \| Operating revenue
Unrealized Mark To Market Results [Abstract]
Total unrealized gains/(losses)	228	(614)	(210)
Commodity contracts \| Cost of operations
Unrealized Mark To Market Results [Abstract]
Total unrealized gains/(losses)	(46)	508	(59)
Interest rate contracts
Unrealized Mark To Market Results [Abstract]
Total unrealized gains/(losses)	$ 9	$ 36	$ 17

-- None. No documentation exists for this element. --

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- Definition

This element represents the amount recorded in the entity's statement of operations for net unrealized gains (losses) on open positions related to trading activities.

+ References

No definition available.

+ Details

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- Definition

Reversal of Previously Unrecognized Unrealized Gain Loss Acquired as Part of Acquisition Trading Activity

+ References

No definition available.

+ Details

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- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to acquired economic hedges.

+ References

No definition available.

+ Details

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- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to economic hedges.

+ References

No definition available.

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- Definition

Amount reversed into entity's statement of operations for previously recognized unrealized (gains) losses on settled positions related to trading activities.

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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-- None. No documentation exists for this element. --

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No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624171-113959

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

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