Earnings Per Share (Net Income Attributable To CME Group) (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net Income Attributable to CME Group	$ 218.0	$ 316.1		$ 729.5	$ 1,066.4
Basic (in shares)	331,377,000	332,289,000		331,091,000	333,449,000
Effect of stock options and restricted stock awards (in shares)	1,081,000	1,048,000		1,056,000	1,058,000
Diluted (in shares)	332,458,000	333,337,000	333,337,000	332,147,000	334,507,000
Earnings per common share, basic (in dollars per share)	$ 0.66	$ 0.95		$ 2.20	$ 3.20
Earnings per common share, diluted (in dollars per share)	$ 0.66	$ 0.95		$ 2.20	$ 3.19
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	4,855,000	5,420,000		4,922,000	4,705,000
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount (in shares)	838,000			850,000
Retained Earnings [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Net Income Attributable to CME Group				$ 729.5	$ 1,066.4