INVESTMENTS - Equity method investments (Details)	Dec. 10, 2019	Mar. 31, 2020 item
Schedule of Equity Method Investments
Number of central counterparties		3
Investment in EuroCCP | Cboe Europe Equities
Schedule of Equity Method Investments
Ownership percentage		20.00%
Number of other investors		4
Remaining ownership percentage acquired	80
Investment in Signal Trading Systems, LLC
Schedule of Equity Method Investments
Ownership percentage		50.00%