INVESTMENTS - Schedule of investments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Schedule of Cost and Equity Method Investments
Equity method investments:		$ 2.0
Other equity investments:	$ 40.7	40.7
Total investments	40.7	42.7
Investment in Signal Trading Systems, LLC
Schedule of Cost and Equity Method Investments
Equity method investments:		$ 2.0
Ownership percentage		50.00%
Investment in Eris Exchange Holdings, LLC
Schedule of Cost and Equity Method Investments
Other equity investments:	20.0	$ 20.0
Investment in American Financial Exchange, LLC
Schedule of Cost and Equity Method Investments
Other equity investments:	10.6	10.6
Investment in Cboe Vest Financial Group, Inc.
Schedule of Cost and Equity Method Investments
Other equity investments:	2.9	2.9
Eris Digital Holdings, LLC
Schedule of Cost and Equity Method Investments
Other equity investments:	0.8	0.8
Investment in OCC
Schedule of Cost and Equity Method Investments
Other equity investments:	$ 0.3	0.3
Investment percentage	20.00%
Other equity investments
Schedule of Cost and Equity Method Investments
Other equity investments:	$ 6.1	$ 6.1