LONG-TERM DEBT AND OTHER FINANCING ARRANGEMENTS Long-term debt and credit facilities, narrative (Details) $ in Thousands, € in Millions		3 Months Ended	6 Months Ended
		Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 EUR (€)
Debt Instrument [Line Items]
Repayments of Long-term Debt			$ 0	$ 113,988
Debt Instrument, Basis Spread on Variable Rate		0.30%
Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.10%
Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate			1.90%
2021 Euro Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 67,600	$ 67,600		€ 60.3
Debt Instrument, Interest Rate, All-In Rate	[1]	3.25%	3.25%		3.25%
2023 Euro Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 524,962	$ 524,962		€ 468.8
Debt Instrument, Interest Rate, All-In Rate	[2]	3.50%	3.50%		3.50%
CME NV and CME BV [Member]
Debt Instrument [Line Items]
Outstanding Shares Pledged, Percentage		100.00%	100.00%		100.00%
CME BV
Debt Instrument [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent		100.00%	100.00%		100.00%

[1] Effective until February 19, 2021. From February 19, 2021 through maturity on April 26, 2023, the rate fixed pursuant to interest rate hedges will increase to 0.97%, with a corresponding decrease in the Guarantee Fee rate, such that the all-in borrowing rate remains 3.50% if our net leverage ratio remains unchanged.

[2] .