INVESTMENTS - Schedule of investments (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Schedule of Cost and Equity Method Investments
Equity Method	$ 24.4	$ 22.1
Other Equity Investments	64.3	60.6
Total Investments	88.7	82.7
Investment in Signal Trading Systems, LLC
Schedule of Cost and Equity Method Investments
Equity Method	14.1	12.5
Investment in EuroCCP
Schedule of Cost and Equity Method Investments
Equity Method	10.3	9.6
Investment in OCC
Schedule of Cost and Equity Method Investments
Other Equity Investments	30.3	30.3
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	5.9	5.9
Other cost method investments
Schedule of Cost and Equity Method Investments
Other Equity Investments	$ 8.1	$ 4.4