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Revenue Recognition - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Capitalized Contract Cost          
Contract cost asset $ 121   $ 121   $ 124
Amortization of contract cost assets $ 24 $ 26 $ 75 $ 79