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Accumulated Other Comprehensive Income (Loss) (Schedule of Accumulated Other Comprehensive Income) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning $ 26,737.9    
Other comprehensive income/(loss) (49.9) $ 77.7 $ (186.8)
Total CME Group Shareholders' Equity, ending 26,486.9 26,737.9  
Accumulated Other Comprehensive Income (Loss) [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning (55.6) (133.3) 53.5
Other comprehensive income/(loss) (49.9) 77.7 (186.8)
Total CME Group Shareholders' Equity, ending (105.5) (55.6) (133.3)
Accumulated Other Comprehensive Income (Loss) [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning (55.6) (133.3) 53.5
Comprehensive Income Before Reclassification Adjustments (48.6) 70.5 (183.2)
Comprehensive Income Reclassification Adjustments (1.3) 6.2 (0.7)
Income tax benefit (expense) 0.0 1.0 (2.9)
Other comprehensive income/(loss) (49.9) 77.7 (186.8)
Total CME Group Shareholders' Equity, ending (105.5) (55.6) (133.3)
Foreign Currency Gain (Loss) [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning (93.8) (174.3) 21.1
Comprehensive Income Before Reclassification Adjustments (61.0) 70.8 (195.4)
Comprehensive Income Reclassification Adjustments 2.2 9.7 0.0
Income tax benefit (expense) 2.2 0.0 0.0
Other comprehensive income/(loss) (56.6) 80.5 (195.4)
Total CME Group Shareholders' Equity, ending (150.4) (93.8) (174.3)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning 62.0 64.7 66.1
Comprehensive Income Before Reclassification Adjustments 0.0 0.0 0.0
Comprehensive Income Reclassification Adjustments (3.6) (3.6) (1.9)
Income tax benefit (expense) 0.9 0.9 0.5
Other comprehensive income/(loss) (2.7) (2.7) (1.4)
Total CME Group Shareholders' Equity, ending 59.3 62.0 64.7
Accumulated Defined Benefit Plans Adjustment [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning (23.4) (22.8) (34.8)
Comprehensive Income Before Reclassification Adjustments 12.2 (0.9) 14.9
Comprehensive Income Reclassification Adjustments 0.1 0.1 1.2
Income tax benefit (expense) (3.0) 0.2 (4.1)
Other comprehensive income/(loss) 9.3 (0.6) 12.0
Total CME Group Shareholders' Equity, ending (14.1) (23.4) (22.8)
Accumulated Net Unrealized Investment Gain (Loss) [Member]      
Accumulated Other Comprehensive Income [Roll Forward]      
Total CME Group Shareholders' Equity, beginning (0.4) (0.9) 1.1
Comprehensive Income Before Reclassification Adjustments 0.2 0.6 (2.7)
Comprehensive Income Reclassification Adjustments 0.0 0.0 0.0
Income tax benefit (expense) (0.1) (0.1) 0.7
Other comprehensive income/(loss) 0.1 0.5 (2.0)
Total CME Group Shareholders' Equity, ending $ (0.3) $ (0.4) $ (0.9)