Earnings Per Share (Net Income Attributable To CME Group) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net Income Attributable to CME Group	$ 359.9	$ 290.0	$ 955.3	$ 820.6
Basic (in shares)	336,323	334,424	336,015	334,144
Dilutive Securities, Effect on EPS	1,816	1,748	1,789	1,676
Diluted (in shares)	338,139	336,172	337,804	335,820
Earnings per common share, basic (in dollars per share)	$ 1.07	$ 0.87	$ 2.84	$ 2.46
Earnings per common share, diluted (in dollars per share)	$ 1.06	$ 0.86	$ 2.83	$ 2.44
Retained Earnings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net Income Attributable to CME Group			$ 955.3	$ 820.6