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Significant Accounting Policies (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | 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 | $ 4,500 | $ 7,400 |
Securities Purchased Under Agreement to Resell Segregated for Regulatory Purposes | 9,200 | 11,000 |
Equity method investments | 23 | 22 |
Cost method investments | $ 11 | $ 33 |
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. |  |
Income (Loss) from Individually Significant Component Disposed of or Held-for-sale, Excluding Discontinued Operations, before Income Tax | $ 11 |  |
Restructuring and Related Cost, Expected Cost | 25 |  |
Supplemental unemployment benefits severance benefits | 3 |  |
Restructuring settlement and impairment provisions | $ 22 |  |
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 |  |

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