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Significant Accounting Policies (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Dec. 31, 2018
May 03, 2007
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 $ 3,900 $ 4,200  
Securities Purchased Under Agreement to Resell Segregated for Regulatory Purposes 14,600 11,400  
Equity method investments 22 23  
Equity Securities without Readily Determinable Fair Value, Amount 5 25  
Equity Securities, FV-NI $ 130 0  
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.    
Exchange Cleared [Member] | Equity Securities [Member]      
Investments at cost $ 5 $ 5  
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