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Investments In Marketable Securities (Details) - USD ($)
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Investments, Debt and Equity Securities [Abstract]      
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Amortized Cost Basis $ 462,000,000    
Available-for-sale Securities, Debt Maturities, Year Two Through Five, Amortized Cost Basis 28,000,000    
Available-for-sale Securities, Realized Losses, Excluding Other than Temporary Impairments (Equal To or Less Than) 1,000,000 $ 1,000,000 $ 1,000,000
Gross proceeds from sales of AFS securities 4,902,000,000 4,569,000,000 4,406,000,000
Other than Temporary Impairment Losses, Investments 0 0 0
Available-for-sale Securities, Gross Unrealized Loss   0 0
Trading Securities, Realized Gain (Loss)- (Equal to or Less Than) $ 1,000,000 $ 1,000,000 $ 1,000,000