Marketable Securities (Narrative) (Details) $ in Millions	Mar. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Investments, Debt and Equity Securities [Abstract]
Available-for-sale Securities, Gross Unrealized Loss, Accumulated in Investments	$ 38	$ 6
Unrealized losses that arose within the last 12 months	$ 35
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	2
Market value of the investments that have been in an unrealized loss position for less than 12 months	$ 267
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value	$ 3
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1