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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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The fair value as of the balance sheet date of firm holdings in debt obligations issued by the US government including short-term Treasury bills, medium-term Treasury notes, and long-term Treasury bonds, as well as debt issued by agencies. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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Useful life of finite-lived intangible assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Liability for amount due employees, in addition to wages and any other money that employers owe employees, when their employment ends through a layoff or other termination. For example, a company may provide involuntarily terminated employees with a lump sum payment equal to one week's salary for every year of employment.

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