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Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Schedule of Trading Securities and Other Trading Assets [Line Items]    
Total trading securities $ 400 $ 384
Trading Securities    
Trading Securities Unrealized Holding Gains (Losses) 51 39
Marketable Securities 296 282
Other Assets $ 104 $ 102