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Fair Value Measurements (Details 4) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
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 $ 175 $ 313 $ 366 $ 820
Realized Gains 9 29 28 50
Realized Losses (11) (29) (27) (44)
Interest and Dividend Income $ 10 $ 10 $ 20 $ 19