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Significant Accounting Policies (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 03, 2007
Increase in net cash from by operating activities		$ 2,356	$ 1,065	$ 635
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		4,200	4,500
Securities Purchased Under Agreement to Resell Segregated for Regulatory Purposes		11,400	9,200
Equity method investments		$ 23	23
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
AOCI gain reclassified to earnings	$ 32
Accumulated tax effect	$ 1
U.S. Statutory Tax Rate		21.00%	35.00%	35.00%
Equity Securities [Member]
Investments at cost		$ 25	$ 5
Exchange Cleared [Member] | Equity Securities [Member]
Investments at cost		5	6
Accounting Standards Update 2016-18 [Member]
Increase in net cash from by operating activities			$ 923	$ 91
Accounting Standards Update 2016-02 [Member]
Change in Accounting Standard, Effect of Adoption, Quantification		$ 136
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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