Clearing Operations (Schedule of Clearing Member Default Fund Contributions And Margin Deposits) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Clearing Operations [Line Items]
Default fund contributions	$ 571
Margin deposits	8,189
Total	8,760
Cash Contributions
Clearing Operations [Line Items]
Default fund contributions	482
Margin deposits	2,896
Total	3,378	$ 2,996
Non-Cash Contributions
Clearing Operations [Line Items]
Default fund contributions	89
Margin deposits	5,293
Total	$ 5,382