Clearing Operations (Narrative) (Details) kr in Millions			1 Months Ended			3 Months Ended		12 Months Ended
	Jan. 27, 2021 USD ($)	Jan. 27, 2021 SEK (kr)	Jan. 31, 2021 USD ($)	Jan. 31, 2021 SEK (kr)	Sep. 30, 2018 USD ($)	Mar. 31, 2021 USD ($) contract fund	Mar. 31, 2020 USD ($) contract	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)
Clearing Operations [Line Items]
Number of member sponsored default funds (in funds) | fund						4
Loss due to commodities market default					$ 133,000,000
Clearing default loss					$ 8,000,000
Capital relief program expense								$ 23,000,000
Expected recovery of initial loss						0.80
Default fund contributions						$ 596,000,000
Default fund contributions and margin deposits						10,283,000,000
Liability due to market default						0
Liability Waterfall
Clearing Operations [Line Items]
Junior capital, cash deposits and pledged assets						37,000,000
Senior capital, cash deposits and pledged assets						23,000,000
Committed capital						79,000,000
Utilize as capital resources
Clearing Operations [Line Items]
Default fund contributions						528,000,000
Utilize as member posted surplus balance
Clearing Operations [Line Items]
Default fund contributions						68,000,000
Nasdaq clearing members cash contributions
Clearing Operations [Line Items]
Default fund contributions						495,000,000
Default fund contributions and margin deposits						$ 3,737,000,000			$ 3,942,000,000
Minimum
Clearing Operations [Line Items]
Reverse purchase agreements, maturity range						1 day
Maximum
Clearing Operations [Line Items]
Reverse purchase agreements, maturity range						15 days
Nasdaq Clearing
Clearing Operations [Line Items]
Administrative fine	$ 34,000,000	kr 300	$ 34,000,000	kr 300
Contingency, accrual						$ 0
Maturity period of time deposits						1 year
Committed capital						$ 139,000,000
Power of assessment of the clearing member's contribution to the financial markets and commodities markets default funds (as a percent)						230.00%
Contract value of resale and repurchase agreements						$ 2,200,000,000	$ 1,000,000,000.0
Total number of derivative contracts cleared | contract						1,434,256	1,410,884