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Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($) disposal | Dec. 31, 2016 USD ($) disposal | Dec. 31, 2015 USD ($) disposal
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Net gain (loss) on disposal of businesses | $ | $ (16) | $ 39 | $ 82
Disposal Group, Not Discontinued Operations |  |  |
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items] |  |  |
Number of dispositions | disposal | 9 | 5 | 7

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- Definition Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84176398&loc=d3e2941-110230 + Details
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