Consolidated Statements Of Shareholders' Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock And Additional Paid-In Capital [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Parent [Member]	Noncontrolling Interest [Member]	Class A Common Stock [Member]	Class A Common Stock [Member] Common Stock And Additional Paid-In Capital [Member]	Class B Common Stock [Member]	Class B Common Stock [Member] Common Stock And Additional Paid-In Capital [Member]
Balance (in shares) at Dec. 31, 2018								356,824		3
Balance at Dec. 31, 2018	$ 25,965.3	$ 21,057.9	$ 4,855.3	$ 5.3	$ 25,918.5
Balance at Dec. 31, 2018						$ 46.8
Net (Income) Loss Attributable to Noncontrolling Interest	1.0					1.0
Net Income (Loss) Attributable to CME Group	496.9		496.9		496.9
Net Income	495.9
Other comprehensive income (loss), net of tax	(4.5)			(4.5)	(4.5)
Dividends on common stock	(268.0)		(268.0)		(268.0)
Cumulative Effect on Retained Earnings, Net of Tax	(6.9)		6.9		6.9
Changes in non-controlling interest due to measurement period adjustments	(16.9)				0.0	(16.9)
Exercise of stock options, shares								64
Exercise of stock options	3.3	3.3			3.3
Vesting of issued restricted Class A common stock, shares								125
Vesting of issued restricted Class A common stock	(13.8)	(13.8)			(13.8)
Stock-based compensation	18.3	18.3			18.3
Balance (in shares) at Mar. 31, 2019								357,013		3
Balance at Mar. 31, 2019	26,186.5	21,065.7	5,091.1	0.8	26,157.6
Balance at Mar. 31, 2019						28.9
Balance (in shares) at Dec. 31, 2019							357,469	357,469	3	3
Balance at Dec. 31, 2019	26,128.9	21,116.8	5,008.7	3.4	26,128.9
Balance at Dec. 31, 2019						30.4
Balance at Dec. 31, 2019	26,159.3
Net (Income) Loss Attributable to Noncontrolling Interest	(0.6)					0.6
Net Income (Loss) Attributable to CME Group	766.2		766.2		766.2
Net Income	766.8
Other comprehensive income (loss), net of tax	(29.6)			(29.6)	(29.6)
Dividends on common stock	(304.7)		(304.7)		(304.7)
Cumulative Effect on Retained Earnings, Net of Tax	(0.3)		(0.3)		(0.3)
Exercise of stock options, shares								55
Exercise of stock options	3.2	3.2			3.2
Vesting of issued restricted Class A common stock, shares								153
Vesting of issued restricted Class A common stock	(19.1)	(19.1)			(19.1)
Stock-based compensation	23.1	23.1			23.1
Balance (in shares) at Mar. 31, 2020							357,677	357,677	3	3
Balance at Mar. 31, 2020	26,567.7	$ 21,124.0	$ 5,469.9	$ (26.2)	$ 26,567.7
Balance at Mar. 31, 2020						$ 31.0
Balance at Mar. 31, 2020	$ 26,598.7