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Summary of Significant Accounting Policies - Other Balance Sheet Information (Details) - USD ($) $ in Millions | Mar. 31, 2018 | Dec. 31, 2017
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Accounting Policies [Abstract] |  |
Accounts receivable allowance for doubtful accounts | $ 27 | $ 28
Property, plant and equipment accumulated depreciation | 4,679 | 4,465
Intangible assets accumulated amortization | 1,393 | 1,818
Out-of-market contracts accumulated amortization | $ 364 | $ 358

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- Definition Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.14) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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