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Investments - Proceeds from available-for-sale securities (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Investments, Debt and Equity Securities [Abstract]        
Gross proceeds from sales $ 18 $ 24 $ 70 $ 52
Gross proceeds from maturities 29 25 96 61
Total $ 47 $ 49 $ 166 $ 113