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Fair Value Investment in Marketable Securities (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Gain Loss On Marketable Securities        
Gross proceeds from sales of AFS securities $ 207 $ 97 $ 507 $ 410