INVESTMENTS - Schedule of investments (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019	Aug. 31, 2019
Schedule of Cost and Equity Method Investments
Equity Method	$ 22.6	$ 22.9
Other Equity Investments	38.7	38.3
Total Investments	61.3	61.2
Investment in Signal Trading Systems, LLC
Schedule of Cost and Equity Method Investments
Equity Method	12.4	12.6
EuroCCP
Schedule of Cost and Equity Method Investments
Equity Method	10.2	10.3
Investment in Eris Exchange Holdings, LLC
Schedule of Cost and Equity Method Investments
Other Equity Investments	20.0	20.8
Investment in American Financial Exchange, LLC
Schedule of Cost and Equity Method Investments
Other Equity Investments	8.6	8.6
Investment in Cboe Vest Financial Group, Inc.
Schedule of Cost and Equity Method Investments
Other Equity Investments	2.9	2.9	$ 2.9
Eris Digital Holdings, LLC
Schedule of Cost and Equity Method Investments
Other Equity Investments	0.8
Investment in OCC
Schedule of Cost and Equity Method Investments
Other Equity Investments	0.3	0.3
Other cost method investments
Schedule of Cost and Equity Method Investments
Other Equity Investments	$ 6.1	$ 5.7