v3.7.0.1
Marketable Securities (Available-for-Sale Debt Securities by Contractual Maturity) (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract]    
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value $ 405  
Available-for-sale Securities, Debt Maturities, Year Two Through Three, Fair Value 383  
Available-for-sale Securities, Debt Securities $ 788 $ 433