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Marketable Securities (Narrative) (Details) - USD ($) $ in Millions | Sep. 30, 2017 | Dec. 31, 2016
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Investments, Debt and Equity Securities [Abstract] |  |
Available-for-sale Securities, Gross Unrealized Loss, Accumulated in Investments | $ 0 | $ 1

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- Definition Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563 + Details
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