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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, 2014
Dec. 31, 2013
Dec. 31, 2012
Investments, Debt and Equity Securities [Abstract]      
Realized gains $ 8us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 6us-gaap_AvailableForSaleSecuritiesGrossRealizedGains $ 20us-gaap_AvailableForSaleSecuritiesGrossRealizedGains
Realized losses $ 5us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses $ 7us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses $ 10us-gaap_AvailableForSaleSecuritiesGrossRealizedLosses