Cash, Cash Equivalents, and Marketable Securities - Gross Grains and Gross Losses Realized on Sales of Available-For-Sale Marketable Securities (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Investments, Debt and Equity Securities [Abstract]
Realized gains	$ 6	$ 20	$ 15
Realized losses	$ (7)	$ 10	$ 11