Clearing Operations (Investment Policy) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Clearing Operations [Line Items]
Total	$ 8,760
Demand deposits
Clearing Operations [Line Items]
Total	1,788	$ 1,328
Central bank certificates
Clearing Operations [Line Items]
Total	666	896
European government debt securities
Clearing Operations [Line Items]
Total	436	508
Reverse repurchase agreements
Clearing Operations [Line Items]
Total	388	116
Supranationals and state owned enterprise debt securities
Clearing Operations [Line Items]
Total	100	148
Nasdaq clearing members cash contributions
Clearing Operations [Line Items]
Total	$ 3,378	$ 2,996