This item represents the carrying amount on the entity's balance sheet of its investment in common stock and member interests in limited liability companies and partnerships of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

No definition available.

Significant Accounting Policies (Details) - USD ($)	3 Months Ended		12 Months Ended
Significant Accounting Policies (Details) - USD ($)	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	May 03, 2007
Increase in net cash from by operating activities	$ 1,201,000,000	$ 779,000,000
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	4,500,000,000		$ 4,500,000,000
Securities Purchased Under Agreement to Resell Segregated for Regulatory Purposes	8,300,000,000		9,200,000,000
Equity method investments	$ 23,000,000		23,000,000
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.
Restructuring and Related Cost, Expected Cost			25,000,000
Restructuring and Related Cost, Incurred Cost	$ 0
Supplemental unemployment benefits severance benefits			3,000,000
Severance costs		1,000,000
Restructuring settlement and impairment provisions			$ 22,000,000
U.S. Statutory Tax Rate	21.00%		35.00%
Equity Securities [Member]
Investments at cost	$ 5,000,000		$ 5,000,000
Exchange Cleared [Member] \| Equity Securities [Member]
Investments at cost	$ 5,000,000		$ 6,000,000
Accounting Standards Update 2016-18 [Member]
Increase in net cash from by operating activities		$ 988,000,000
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