v3.7.0.1
Marketable Securities Narrative (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]    
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value $ 485 $ 265
Trading Securities $ 17 $ 13