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Marketable and Other Securities - Narrative (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Marketable Securities [Abstract]        
Sale proceeds from investments classified as available-for-sale and sold prior to maturity $ 557 $ 554 $ 1,100 $ 920