Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net Income	$ 766.8	$ 495.9
Other comprehensive income (loss), net of tax:
Net unrealized holding gains (losses) arising during the period	(0.6)	0.8
Income tax benefit (expense)	0.2	(0.2)
Investment securities, net	(0.4)	0.6
Net change in defined benefit plans arising during the period	(2.0)	(2.7)
Amortization of net actuarial (gains) losses included in compensation and benefits expense	1.2	1.2
Income tax benefit (expense)	0.2	0.4
Defined benefit plans, net	(0.6)	(1.1)
Net unrealized holding gains (losses) during the period	0.0	0.3
Reclassification of net unrealized (gains) losses to interest expense and other non-operating income (expense)	(1.8)	(0.3)
Income tax benefit (expense)	0.4	0.0
Derivative investments, net	(1.4)	0.0
Foreign currency translation adjustments	(27.8)	(4.0)
Reclassification of net (gains) losses from foreign currency entities to other expenses	0.6	0.0
Income tax benefit (expense)	0.0	0.0
Foreign currency translation, net	(27.2)	(4.0)
Other comprehensive income (loss), net of tax	(29.6)	(4.5)
Comprehensive Income	737.2	491.4
Comprehensive (Income) Loss Attributable to Noncontrolling Interest	(0.6)	1.0
Comprehensive Income attributable to CME Group	$ 736.6	$ 492.4