Clearing Operations (Schedule of Derivative Contracts) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) TWh contract	Dec. 31, 2019 TWh contract
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 1,138
Total number of cleared contracts (in contracts) | contract	93,101,056	93,380,173
Total volume in cleared power, in Terawatt hours (TWh) | TWh	956	842
Commodity and seafood options, futures and forwards
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 122
Total number of cleared contracts (in contracts) | contract	672,219	542,557
Fixed-income options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 773
Total number of cleared contracts (in contracts) | contract	21,299,713	21,464,522
Stock options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 175
Total number of cleared contracts (in contracts) | contract	19,757,733	23,777,980
Index options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 68
Total number of cleared contracts (in contracts) | contract	51,371,391	47,595,114