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Fair Value Measurements (Details 4) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Proceeds from the sale of investments in available-for-sale securities, realized gains and losses on those sales, and interest and dividend income    
Sale Proceeds $ 153 $ 191
Realized Gains 7 19
Realized Losses (6) (16)
Interest and Dividend Income $ 9 $ 10