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Acquisitions and Divestiture (2018 Divestiture) (Details) - USD ($)
$ in Millions
1 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Net gain on divestiture of businesses     $ 33 $ 0 $ 0
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Public Relations Solutions and Digital Media Services          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Net gain on divestiture of businesses   $ 33      
Gain on divestiture of business, after tax   $ 14      
Working capital adjustment $ 8        
Working capital adjustment, after tax $ 5