Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 359.9	$ 290.0	$ 955.3	$ 820.4
Other comprehensive income, net of tax
Net unrealized holding gains arising during the period	(100.2)	(66.6)	(91.0)	(10.7)
Reclassification adjustment for gains included in investment income	2.6	0.0	(3.4)	0.0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	0.0	(5.0)	1.2	(6.5)
Investment securities, net	(97.6)	(71.6)	(93.2)	(17.2)
Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net Unamortized Gain (Loss) Arising During Period, Net of Tax	0.0	0.0	(0.3)	(3.2)
Amortization of net actuarial (gains) losses included in pension expense	0.6	0.0	2.0	0.2
Defined benefit plans, income tax benefit (expense)	(0.2)	0.0	(0.6)	1.1
Defined benefit plans, net	0.4	0.0	1.1	(1.9)
Net unrealized holding losses arising during the period	0.0	0.0	(4.7)	0.0
Loss on Cash Flow Hedge Ineffectiveness	0.0	0.0	1.8	0.0
Other Comprehensive Income (Loss), Reclassification Adjustment on Derivatives Included in Net Income, before Tax	(0.3)	(0.4)	(0.9)	(1.1)
Derivative instruments, income tax benefit (expense)	0.1	0.1	1.5	0.4
Derivative instruments, net	(0.2)	(0.3)	(2.3)	(0.7)
Foreign currency translation adjustment	(3.4)	(6.9)	(10.0)	(2.7)
Other Comprehensive Income (Loss), Foreign Currency Translation Adjustment, Tax	1.1	2.6	3.6	1.0
Foreign currency translation, net	(2.3)	(4.3)	(6.4)	(1.7)
Other Comprehensive Income (Loss), Net of Tax	(99.7)	(76.2)	(100.8)	(21.5)
Comprehensive income	260.2	213.8	854.5	798.9
Less: comprehensive income attributable to redeemable non-controlling interest	0.0	0.0	0.0	(0.2)
Comprehensive income attributable to CME Group	$ 260.2	$ 213.8	$ 854.5	$ 799.1