Earnings Per Share (Net Income Attributable To CME Group) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Earnings Per Share [Abstract]
Net Income Attributable to CME Group	$ 503.3	$ 513.8	$ 1,269.5	$ 1,010.7
Basic (in shares)	357,691	357,060	357,607	356,973
Effect of stock options, restricted stock and performance shares	766	1,095	846	1,130
Diluted (in shares)	358,457	358,155	358,453	358,103
Earnings per common share, basic (in dollars per share)	$ 1.41	$ 1.44	$ 3.55	$ 2.83
Earnings per common share, diluted (in dollars per share)	$ 1.40	$ 1.43	$ 3.54	$ 2.82