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Summary of Significant Accounting Policies Equity investment and excess costs (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Sep. 01, 2016
Schedule of Equity Method Investments [Line Items] |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life | 16 years |  |
Sale Equity Interest in SNG [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Disposal Group, Equity Interest Sold |  |  | 50.00%
Acquisition-related Costs [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life | 14 years |  |
Property, Plant and Equipment, Other Types [Member] | Amortized [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method of Accounting and Excess Investment Cost | $ 732 | $ 767 |
Goodwill [Member] | Unamortization [Member] |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |
Equity Method of Accounting and Excess Investment Cost | $ 956 | $ 956 |

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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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