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Goodwill and Acquired Intangible Assets (Narrative) (Details) - USD ($)
1 Months Ended 3 Months Ended 9 Months Ended
Apr. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Indefinite-lived Intangible Assets [Line Items]            
Amount of goodwill expected to be deductible for tax purposes   $ 749,000,000   $ 749,000,000    
Goodwill, impairment loss       0 $ 0  
Amortization expense for purchased finite-lived intangible assets   27,000,000 $ 22,000,000 83,000,000 $ 67,000,000  
Foreign currency translation amortization expense   1,070,000,000   1,070,000,000   $ 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   $ (81,000,000)   $ (81,000,000)   $ (65,000,000)