Clearing Operations (Schedule of Clearing Member Default Fund Contributions And Margin Deposits) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Clearing Operations [Line Items]
Default fund contributions	$ 505
Margin deposits	6,794
Total	7,299
Cash Contributions
Clearing Operations [Line Items]
Default fund contributions	414
Margin deposits	2,827
Total	3,241	$ 2,996
Non-Cash Contributions
Clearing Operations [Line Items]
Default fund contributions	91
Margin deposits	3,967
Total	$ 4,058