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Investments - Proceeds from available-for-sale securities (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]        
Gross proceeds from sales $ 24 $ 15 $ 52 $ 54
Gross proceeds from maturities $ 25 $ 9 $ 61 $ 33