Clearing Operations (Schedule of Derivative Contracts) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2020 USD ($) TWh contract	Sep. 30, 2019 TWh contract
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 1,223
Total number of cleared contracts (in contracts) | contract	72,458,426	69,956,891
Total volume in cleared power, in Terawatt hours (TWh) | TWh	670	614
Commodity and seafood options, futures and forwards
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 163
Total number of cleared contracts (in contracts) | contract	476,480	393,153
Fixed-income options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 792
Total number of cleared contracts (in contracts) | contract	16,456,583	15,780,707
Stock options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 168
Total number of cleared contracts (in contracts) | contract	14,322,903	18,030,431
Index options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 100
Total number of cleared contracts (in contracts) | contract	41,202,460	35,752,600