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Revenue (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Revenue [Abstract]          
Capitalized Contract Cost, Net $ 18   $ 18   $ 21
Capitalized Contract Cost, Amortization 6 $ 6 16 $ 17  
Capitalized Contract Cost, Impairment Loss     0    
Deferred revenue 180   180   185
Contract asset 37   37   $ 0
Revenue recognized $ 40   $ 184