INVESTMENTS - Schedule of investments (Details) - USD ($) $ in Millions	6 Months Ended
Jun. 30, 2021	Dec. 31, 2020
Schedule of Cost and Equity Method Investments
Equity method investments:		$ 2.0
Other equity investments:	$ 41.1	40.7
Total investments	41.1	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:	1.2	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

The percentage of equity investment securities.

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[Line Items] for Schedule of Cost and Equity Method Investments [Table]

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The percentage of ownership of common stock or equity participation in the investee accounted for under the equity method of accounting.

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This item represents the carrying amount on the entity's balance sheet of its investment in common stock of an equity method investee. This is not an indicator of the fair value of the investment, rather it is the initial cost adjusted for the entity's share of earnings and losses of the investee, adjusted for any distributions (dividends) and other than temporary impairment (OTTI) losses recognized.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of investment in equity method investee and investment in and advance to affiliate.

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