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Financial Assets And Financial Liabilities (Narrative) (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities purchased under agreement to resell segregated for regulatory purposes $ 8,300 $ 9,200
Total financial instruments owned, at fair value 2,415 3,154
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total financial instruments owned, at fair value 4 $ 4
Level 3 | Corporate And Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total financial instruments owned, at fair value 3  
Level 3 | Common Stock [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total financial instruments owned, at fair value $ 1