Earnings Per Share (Net Income Attributable To CME Group) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Earnings Per Share [Abstract]
Net Income (Loss) Attributable to CME Group	$ 766.2	$ 496.9
Basic (in shares)	357,524	356,886
Effect of stock options, restricted stock and performance shares	931	1,161
Diluted (in shares)	358,455	358,047
Earnings per common share, basic (in dollars per share)	$ 2.14	$ 1.39
Earnings per common share, diluted (in dollars per share)	$ 2.14	$ 1.39