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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, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Unrealized Mark To Market Results [Abstract] |  |  |
Reversal of previously recognized unrealized (gains)/losses on settled positions related to economic hedges | $ (73) | $ 47 | $ (128)
Reversal of acquired gain positions related to economic hedges | (10) | 0 | (12)
Net unrealized gains on open positions related to economic hedges | 97 | 159 | 12
Total unrealized mark-to-market gains/(losses) for economic hedging activities | 14 | 206 | (128)
Reversal of previously recognized unrealized (gains)/losses on settled positions related to trading activity | (12) | (25) | 10
Net unrealized gains on open positions related to trading activity | 29 | 14 | 18
Total unrealized mark-to-market gains/(losses) for trading activity | 17 | (11) | 28
Total unrealized gains/(losses) | 31 | 195 | (100)
Credit Risk Related Contingent Features [Abstract] |  |  |
Collateral required contracts with credit rating contingent features in a net liability position | 11 |  |
Adequate Assurance Clauses |  |  |
Credit Risk Related Contingent Features [Abstract] |  |  |
Collateral required for contracts in net liability positions that have adequate assurance clauses | 16 |  |
Credit Rating Contingent Features |  |  |
Credit Risk Related Contingent Features [Abstract] |  |  |
Collateral required for contracts in net liability positions that have adequate assurance clauses | 14 |  |
Commodity contracts |  |  |
Unrealized Mark To Market Results [Abstract] |  |  |
Total unrealized gains/(losses) | 31 | 195 | (100)
Commodity contracts | Operating revenue |  |  |
Unrealized Mark To Market Results [Abstract] |  |  |
Total unrealized gains/(losses) | (113) | 241 | (608)
Commodity contracts | Cost of operations |  |  |
Unrealized Mark To Market Results [Abstract] |  |  |
Total unrealized gains/(losses) | 144 | (46) | 508
Interest rate contracts |  |  |
Unrealized Mark To Market Results [Abstract] |  |  |
Total unrealized gains/(losses) | $ 0 | $ 4 | $ (8)

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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 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. + 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 trading activities. + References No definition available. + Details
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- Definition Unrealized Gain (Loss) on Derivatives, Mark To Market, Economic Hedging Activities + References No definition available. + Details
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- Definition Unrealized Gain (Loss) on Derivatives, Mark To Market, Trading Activities + References No definition available. + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959 + Details
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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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4CC -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL109998890-113959 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4C -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624171-113959 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4H -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959 + Details
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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585 + Details
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