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Revenue (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Revenue from Contract with Customer [Abstract]      
Revenue recognized from performance obligation satisfied in previous period $ 9 $ 18 $ 37
Contract liability, revenue recognized   $ 144