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Investments - Narrative (Details)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2019
USD ($)
Sep. 30, 2019
USD ($)
Investments, Debt and Equity Securities [Abstract]    
Gross realized investment gains $ 11 $ 11
Held-to-maturity, within one year, amortized cost $ 79 $ 79