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Accounting for Derivative Instruments and Hedging Activities - Fair Value Within the Derivative Instrument Valuation (Details) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Derivative [Line Items] |  |
Derivative assets | $ 798 | $ 1,248
Derivative liabilities | 752 | 1,376
Designated as Hedging Instrument |  |
Derivative [Line Items] |  |
Derivative assets | 12 | 12
Derivative liabilities | 16 | 69
Designated as Hedging Instrument | Interest rate contracts current |  |
Derivative [Line Items] |  |
Derivative assets | 1 | 0
Derivative liabilities | 5 | 28
Designated as Hedging Instrument | Interest rate contracts long-term |  |
Derivative [Line Items] |  |
Derivative assets | 11 | 12
Derivative liabilities | 11 | 41
Not Designated as Hedging Instrument |  |
Derivative [Line Items] |  |
Derivative assets | 786 | 1,236
Derivative liabilities | 736 | 1,307
Not Designated as Hedging Instrument | Interest rate contracts current |  |
Derivative [Line Items] |  |
Derivative assets | 9 | 0
Derivative liabilities | 15 | 7
Not Designated as Hedging Instrument | Interest rate contracts long-term |  |
Derivative [Line Items] |  |
Derivative assets | 32 | 37
Derivative liabilities | 28 | 12
Not Designated as Hedging Instrument | Commodity contracts current |  |
Derivative [Line Items] |  |
Derivative assets | 616 | 1,067
Derivative liabilities | 535 | 1,057
Not Designated as Hedging Instrument | Commodity contracts long-term |  |
Derivative [Line Items] |  |
Derivative assets | 129 | 132
Derivative liabilities | $ 158 | $ 231

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- Definition 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. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 50 -Paragraph 4B -Subparagraph (a),(c) -URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624163-113959 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226000-175313 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 20 -Section 50 -Paragraph 4D -Subparagraph (c)(2) -URI http://asc.fasb.org/extlink&oid=82924980&loc=SL51823488-111719 + Details
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