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Goodwill and Acquired Intangible Assets (Narrative) (Details) - USD ($)
 1 Months Ended 12 Months Ended
Oct. 31, 2018
Apr. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Indefinite-lived Intangible Assets [Line Items]          
Measurement period adjustment     $ (8,000,000)    
Amount of goodwill expected to be deductible for tax purposes     807,000,000    
Goodwill, impairment loss     0 $ 0 $ 0
Non-cash write-off related to indefinite-lived intangible asset     0 0 0
Amortization expense of acquired intangible assets     109,000,000 92,000,000 $ 82,000,000
Foreign currency translation amortization expense     1,045,000,000 1,173,000,000  
Disposal Group, Disposed of by Sale, Not Discontinued Operations | Public Relations Solutions and Digital Media Services | Trade name          
Indefinite-lived Intangible Assets [Line Items]          
Non-cash write-off related to indefinite-lived intangible asset   $ 2,000,000      
Foreign currency translation adjustment          
Indefinite-lived Intangible Assets [Line Items]          
Foreign currency translation amortization expense     (85,000,000) $ (65,000,000)  
Information Services          
Indefinite-lived Intangible Assets [Line Items]          
Measurement period adjustment $ 8,000,000   $ (8,000,000)