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Fair Value of Financial Instruments - Derivative Fair Value Measurements, Narrative (Details) - USD ($) | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Fair Value Disclosures [Abstract] |  |
Total derivative assets valued with prices provied by models and other valuation techniques (as a percent) | 39.00% |
Total derivative liabilities valued with prices provied by models and other valuation techniques (as a percent) | 35.00% |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |
Credit result of reserve change to fair value | $ (4,000,000) | $ 0
Commodity contracts |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |
Loss included in credit reserve | 1,000,000 |
Interest rate contracts |  |
Valuation and Qualifying Accounts Disclosure [Line Items] |  |
Loss included in credit reserve | $ 3,000,000 |

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- Definition This element represents the portion of total fair value of all derivative assets, which are valued using various valuation techniques including internal models based on a fundamental analysis of the market and extrapolation of observable market data with similar characteristics. + References No definition available. + Details
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- Definition Amount of increase (decrease) in the allowances and reserves, which consist of the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs, from adjustments. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SX 210.12-09) -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Amount of increase (decrease) in allowances and reserves, the valuation and qualifying accounts that are either netted against the cost of an asset or that reflect a liability established to represent expected future costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24092-122690 + Details
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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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