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Investments In Marketable Securities (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2015
Sep. 30, 2014
Investments, Debt and Equity Securities [Abstract]        
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis $ 430,000,000   $ 430,000,000  
Available-for-sale Securities, Debt Securities, Noncurrent 33,000,000   33,000,000  
Gain Loss On Marketable Securities        
Realized losses on the sale of available-for-sale securities 0 $ 0 0 $ 0
Other-than-temporary impairment of marketable securities 0 0 0 0
Gross proceeds from sales of AFS securities 1,224,000,000 1,144,000,000 3,705,000,000 3,362,000,000
Available-for-sale Securities, Gross Realized Gain (Loss)- (Less than) 1,000,000 1,000,000 1,000,000 1,000,000
Trading Securities, Realized Gain (Loss)- (Less than) $ 1,000,000 $ 1,000,000 $ 1,000,000 $ 1,000,000