Clearing Operations (Schedule of Derivative Contracts) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2020 USD ($) TWh contract	Mar. 31, 2019 TWh contract
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 1,577
Total number of cleared contracts (in contracts) | contract	31,126,294	24,148,465
Total volume in cleared power, in Terawatt hours (TWh) | TWh	292	250
Commodity and seafood options, futures and forwards
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 382
Total number of cleared contracts (in contracts) | contract	198,870	146,249
Fixed-income options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 711
Total number of cleared contracts (in contracts) | contract	5,842,771	5,404,135
Stock options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 312
Total number of cleared contracts (in contracts) | contract	6,089,250	6,854,813
Index options and futures
Clearing Operations [Line Items]
Market value of derivative contracts | $	$ 172
Total number of cleared contracts (in contracts) | contract	18,995,403	11,743,268