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Investments (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Schedule of Trading Securities [Line Items]      
Trading Securities, Debt $ 48   $ 47
Trading Securities, Equity 64   60
Trading Securities, Total 112   $ 107
Trading Securities, Recognized Gain 4 $ 5  
Trading Securities, Recognized (Loss) 0 (4)  
Trading Securities, Recognized Net Gain (Loss) $ 4 $ 1