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Investments - Available-for-sale (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]      
Cash $ 802 $ 677  
Total realized gains 37 92 $ 29
Total realized losses $ 4 $ 36 $ 394