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
Less: net (income) loss attributable to non-controlling interests	0.2					0.2
Net Income Attributable to CME Group	1,010.7		1,010.7		1,010.7
Net Income	1,010.5
Other comprehensive income (loss), net of tax	0.3			0.3	0.3
Dividends, Cash	(536.1)		(536.1)		(536.1)
Impact of accounting standards update adoption	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								162
Exercise of stock options	7.0	7.0			7.0
Vesting of issued restricted Class A common stock, shares								128
Vesting of issued restricted Class A common stock	(14.1)	(14.1)			(14.1)
Stock Issued During Period, Shares, Other								16
Stock Issued During Period, Value, Other	3.1	3.1			3.1
Stock Issued During Period, Shares, Employee Stock Purchase Plans								13
Stock Issued During Period, Value, Employee Stock Purchase Plan	2.5	2.5			2.5
Stock-based compensation	56.4	56.4			56.4
Balance (in shares) at Jun. 30, 2019								357,143		3
Balance at Jun. 30, 2019	26,484.9	21,112.8	5,336.8	5.6	26,455.2
Balance at Jun. 30, 2019						29.7
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
Less: net (income) loss attributable to non-controlling interests	(0.8)					(0.8)
Net Income Attributable to CME Group	513.8		513.8		513.8
Net Income	514.6
Other comprehensive income (loss), net of tax	4.8			4.8	4.8
Dividends, Cash	(268.1)		(268.1)		(268.1)
Exercise of stock options, shares								98
Exercise of stock options	3.7	3.7			3.7
Vesting of issued restricted Class A common stock, shares								3
Vesting of issued restricted Class A common stock	(0.3)	(0.3)			(0.3)
Stock Issued During Period, Shares, Other								16
Stock Issued During Period, Value, Other	3.1	3.1			3.1
Stock Issued During Period, Shares, Employee Stock Purchase Plans								13
Stock Issued During Period, Value, Employee Stock Purchase Plan	2.5	2.5			2.5
Stock-based compensation	38.1	38.1			38.1
Balance (in shares) at Jun. 30, 2019								357,143		3
Balance at Jun. 30, 2019	26,484.9	21,112.8	5,336.8	5.6	26,455.2
Balance at Jun. 30, 2019						29.7
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
Less: net (income) loss attributable to non-controlling interests	(0.6)					0.6
Net Income Attributable to CME Group	1,269.5		1,269.5		1,269.5
Net Income	1,270.1
Other comprehensive income (loss), net of tax	(16.5)			(16.5)	(16.5)
Dividends, Cash	(609.4)		(609.4)		(609.4)
Impact of accounting standards update adoption	(0.3)				(0.3)
Exercise of stock options, shares								66
Exercise of stock options	3.8	3.8			3.8
Vesting of issued restricted Class A common stock, shares								158
Vesting of issued restricted Class A common stock	(19.4)	(19.4)			(19.4)
Stock Issued During Period, Shares, Other								17
Stock Issued During Period, Value, Other	2.9	2.9			2.9
Stock Issued During Period, Shares, Employee Stock Purchase Plans								17
Stock Issued During Period, Value, Employee Stock Purchase Plan	2.9	2.9			2.9
Stock-based compensation	45.5	45.5			45.5
Balance (in shares) at Jun. 30, 2020							357,727	357,727	3	3
Balance at Jun. 30, 2020	26,807.9	21,152.5	5,668.5	(13.1)	26,807.9
Balance at Jun. 30, 2020						31.0
Balance at Jun. 30, 2020	26,838.9
Balance (in shares) at Mar. 31, 2020								357,677		3
Balance at Mar. 31, 2020		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
Less: net (income) loss attributable to non-controlling interests	0.0					0.0
Net Income Attributable to CME Group	503.3		503.3		503.3
Net Income	503.3
Other comprehensive income (loss), net of tax	13.1			13.1	13.1
Dividends, Cash	(304.7)		(304.7)		(304.7)
Exercise of stock options, shares								11
Exercise of stock options	0.6	0.6			0.6
Vesting of issued restricted Class A common stock, shares								5
Vesting of issued restricted Class A common stock	(0.3)	(0.3)			(0.3)
Stock Issued During Period, Shares, Other								17
Stock Issued During Period, Value, Other	2.9	2.9			2.9
Stock Issued During Period, Shares, Employee Stock Purchase Plans								17
Stock Issued During Period, Value, Employee Stock Purchase Plan	2.9	2.9			2.9
Stock-based compensation	22.4	22.4			22.4
Balance (in shares) at Jun. 30, 2020							357,727	357,727	3	3
Balance at Jun. 30, 2020	26,807.9	$ 21,152.5	$ 5,668.5	$ (13.1)	$ 26,807.9
Balance at Jun. 30, 2020						$ 31.0
Balance at Jun. 30, 2020	$ 26,838.9