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Revenue Recognition - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2018
Revenue from Contract with Customer [Abstract]        
Amounts recorded gross in revenues that are billed to customers and remitted to governmental authorities $ 90,000,000 $ 80,000,000 $ 85,000,000  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Cumulative effect of accounting changes   195,000,000    
Bad debts expense 101,000,000 95,000,000 $ 102,000,000  
Capitalized contract cost        
Amortization of contract cost assets 124,000,000      
Impairment of capitalized contract cost $ 0      
Minimum        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Capitalized contract cost, amortization period 14 months      
Maximum        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Capitalized contract cost, amortization period 5 years      
Retained earnings        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Cumulative effect of accounting changes   $ 165,000,000    
Revenue from contract with customer        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Bad debts expense $ 100,000,000      
ASU 2014-09 | Retained earnings        
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]        
Cumulative effect of accounting changes       $ 164,000,000