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Significant Accounting Policies (Details) - USD ($)
 3 Months Ended 9 Months Ended 12 Months Ended
Jun. 30, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
May 03, 2007
Increase in net cash from by operating activities   $ 2,764,000,000 $ 2,295,000,000    
SIP expense - Year of grant         50.00%
SIP expense - Remaining vesting period         50.00%
SIP expense - Employees over 59 in year of grant         100.00%
Percent of shares cancelled post employment         50.00%
Over 59 percent of shares eligible         100.00%
U.S. Treasury Securities   4,100,000,000   $ 4,500,000,000  
Securities Purchased Under Agreement to Resell Segregated for Regulatory Purposes   8,700,000,000   9,200,000,000  
Equity method investments   $ 23,000,000   23,000,000  
Property and equipment useful lives, description   Computer equipment is depreciated over three to five years and office furniture and equipment are depreciated over five to seven years. Intangible assets with a finite life are amortized on a straight line basis over their estimated useful lives of three years, and tested for recoverability whenever events indicate that the carrying amounts may not be recoverable. Qualifying costs for internally developed software are capitalized and amortized over the expected useful life of the developed software, not to exceed three years.      
Restructuring and Related Cost, Incurred Cost   $ 0   25,000,000  
Severance costs     3,000,000 3,000,000  
Restructuring settlement and impairment provisions       $ 22,000,000  
Restructuring costs and asset impairment charges     21,000,000    
AOCI gain reclassified to earnings $ 32,000,000        
Accumulated tax effect $ 1,000,000        
U.S. Statutory Tax Rate   21.00%   35.00%  
Equity Securities [Member]          
Investments at cost   $ 26,000,000   $ 5,000,000  
Exchange Cleared [Member] | Equity Securities [Member]          
Investments at cost   $ 5,000,000   $ 6,000,000  
Accounting Standards Update 2016-18 [Member]          
Increase in net cash from by operating activities     $ 1,922,000,000    
Maximum [Member] | Computer Equipment [Member]          
Property and equipment useful lives   5 years      
Maximum [Member] | Office Furniture And Equipment [Member]          
Property and equipment useful lives   7 years      
Maximum [Member] | Internally Developed Software [Member]          
Intangible assets useful lives   3 years      
Maximum [Member] | Finite-Lived Intangible Assets [Member]          
Intangible assets useful lives   3 years      
Minimum [Member] | Computer Equipment [Member]          
Property and equipment useful lives   3 years      
Minimum [Member] | Office Furniture And Equipment [Member]          
Property and equipment useful lives   5 years