INVESTMENTS - Equity method investments (Details)	Jul. 01, 2020	Jun. 30, 2020 item
Schedule of Equity Method Investments
Number of central counterparties		3
EuroCCP | Cboe Europe Limited
Schedule of Equity Method Investments
Ownership percentage		20.00%
Number of other investors		4
Investment in Signal Trading Systems, LLC
Schedule of Equity Method Investments
Ownership percentage		50.00%
Subsequent Event | EuroCCP | Cboe Europe Limited
Schedule of Equity Method Investments
Remaining ownership percentage acquired	80