Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Statement of Comprehensive Income [Abstract]
Net Income	$ 503.3	$ 514.6	$ 1,270.1	$ 1,010.5
Other comprehensive income (loss), net of tax:
Net unrealized holding gains (losses) arising during the period	1.4	0.9	0.8	1.7
Reclassification of net (gains) losses on sales included in investment income	0.0	(0.1)	0.0	(0.1)
Income tax benefit (expense)	(0.4)	(0.2)	(0.2)	(0.4)
Investment securities, net	1.0	0.6	0.6	1.2
Net change in defined benefit plans arising during the period	0.0	0.0	(2.0)	(2.7)
Amortization of net actuarial (gains) losses included in compensation and benefits expense	1.1	1.2	2.3	2.4
Income tax benefit (expense)	(0.3)	(0.3)	(0.1)	0.1
Defined benefit plans, net	0.8	0.9	0.2	(0.2)
Net unrealized holding gains (losses) arising during the period	0.0	(0.5)	0.0	(0.2)
Reclassification of net unrealized (gains) losses to interest expense and other non-operating income (expense)	(0.3)	(0.3)	(2.1)	(0.6)
Income tax benefit (expense)	0.0	0.1	0.4	0.1
Derivative investments, net	(0.3)	(0.7)	(1.7)	(0.7)
Foreign currency translation adjustments	11.6	1.0	(16.2)	(3.0)
Reclassification of net currency (gains) losses from foreign entities to other expenses	0.0	0.0	0.6	0.0
Income tax benefit (expense)	0.0	(3.0)	0.0	(3.0)
Foreign currency translation, net	11.6	4.0	(15.6)	0.0
Other comprehensive income (loss), net of tax	13.1	4.8	(16.5)	0.3
Comprehensive income	516.4	519.4	1,253.6	1,010.8
Less: comprehensive (income) loss attributable to non-controlling interests	0.0	(0.8)	(0.6)	0.2
Comprehensive income attributable to CME Group	$ 516.4	$ 518.6	$ 1,253.0	$ 1,011.0