LONG-TERM DEBT AND OTHER FINANCING ARRANGEMENTS Long-term debt and credit facilities, narrative (Details) $ in Thousands, € in Millions		3 Months Ended
		Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Mar. 31, 2020 EUR (€)
Debt Instrument [Line Items]
Repayments of Long-term Debt		$ 0	$ 68,928
Debt Instrument, Basis Spread on Variable Rate		1.45%
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		$ 66,103		€ 60.3
Debt Instrument, Interest Rate, All-In Rate	[1]	3.25%		3.25%
2023 Euro Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 513,600		€ 468.8
Debt Instrument, Interest Rate, All-In Rate	[2]	3.50%		3.50%
CME NV and CME BV [Member]
Debt Instrument [Line Items]
Outstanding Shares Pledged, Percentage		100.00%		100.00%
CME BV
Debt Instrument [Line Items]
Noncontrolling Interest, Ownership Percentage by Parent		100.00%		100.00%

[1]   Base Rate
  Rate Fixed Pursuant to Interest Rate Hedges
  Guarantee Fee Rate
  All-in Borrowing Rate
2021 Euro Loan 1.28 %   0.47 %   1.50 %   3.25 % 2023 Euro Loan 1.28 %   0.28 % ⁽¹⁾  1.94 %   3.50 % 2023 Revolving Credit Facility (if drawn) 4.70 % ⁽²⁾  — %   — %   4.70 %

[2] 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.