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Significant Accounting Policies (Narrative) (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended | 24 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2020 | Sep. 30, 2019 | Sep. 30, 2020 | Sep. 30, 2019 | Dec. 31, 2019 | May 03, 2007
Significant Accounting Policies [Line Items] |  |  |  |  |  |
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%
Other Income Loss | $ 29 | $ (47) | $ 25 | $ (2) |  |
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. |  |  |
Restatement Adjustment [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Other fees and services |  | 35 |  | 105 |  |
Other Income Loss |  | $ 7 |  | $ 20 |  |
Maximum [Member] | Market Making [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Percent of segment's net revenue |  |  |  |  | 7.00% |
Percent of segment's income before income taxes |  |  |  |  | 4.00% |
Percent of segment's total assets |  |  |  |  | 6.00% |
Maximum [Member] | Computer Equipment [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Property and equipment useful lives |  |  | 5 years |  |  |
Maximum [Member] | Office Furniture And Equipment [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Property and equipment useful lives |  |  | 7 years |  |  |
Maximum [Member] | Internally Developed Software [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Intangible assets useful lives |  |  | 3 years |  |  |
Maximum [Member] | Finite-Lived Intangible Assets [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Intangible assets useful lives |  |  | 3 years |  |  |
Minimum [Member] | Computer Equipment [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Property and equipment useful lives |  |  | 3 years |  |  |
Minimum [Member] | Office Furniture And Equipment [Member] |  |  |  |  |  |
Significant Accounting Policies [Line Items] |  |  |  |  |  |
Property and equipment useful lives |  |  | 5 years |  |  |

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