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Fair Value Measurements (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Fair Value Disclosures [Abstract]        
Gain on sale of marketable securities $ 18 $ 25 $ 35 $ 65