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Investments (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Trading Securities [Line Items]          
Trading Securities, Debt $ 42   $ 42   $ 47
Trading Securities, Equity 60   60   60
Trading Securities, Total 102   102   $ 107
Trading Securities, Recognized Gain 3 $ 4 10 $ 11  
Trading Securities, Recognized (Loss) 0 0 0 (4)  
Trading Securities, Recognized Net Gain (Loss) $ 3 $ 4 $ 10 $ 7