Condensed Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Total revenues	$ 868.7	$ 620.6	$ 1,790.2	$ 1,222.1
Cost of revenues:
Total cost of revenues	571.8	337.4	1,135.0	659.5
Revenues less cost of revenues	296.9	283.2	655.2	562.6
Operating expenses:
Compensation and benefits	54.9	52.2	108.2	100.3
Depreciation and amortization	38.0	43.7	78.5	90.9
Technology support services	12.5	11.8	24.4	23.7
Professional fees and outside services	12.3	19.2	27.2	35.4
Travel and promotional expenses	0.9	3.0	3.0	5.6
Facilities costs	4.1	3.0	8.2	5.1
Acquisition-related costs	9.4	20.8	10.2	23.1
Other expenses	3.1	4.3	7.4	7.9
Total operating expenses	135.2	158.0	267.1	292.0
Operating income	161.7	125.2	388.1	270.6
Non-operating (expenses) income:
Interest expense, net	(7.3)	(10.0)	(14.6)	(19.9)
Other income (expense), net	2.2	4.4	0.6	(4.4)
Income before income tax provision	156.6	119.6	374.1	246.3
Income tax provision	43.0	35.1	103.1	67.7
Net income	113.6	84.5	271.0	178.6
Net loss attributable to redeemable noncontrolling interest		3.8		4.0
Net income excluding redeemable noncontrolling interest	113.6	88.3	271.0	182.6
Change in redemption value of redeemable noncontrolling interest		(0.2)		(0.4)
Net income allocated to participating securities	(0.3)	(0.5)	(0.7)	(1.1)
Net income allocated to common stockholders	$ 113.3	$ 87.6	$ 270.3	$ 181.1
Basic earnings per share (in dollars per share)	$ 1.04	$ 0.79	$ 2.46	$ 1.62
Diluted earnings per share (in dollars per share)	$ 1.03	$ 0.78	$ 2.45	$ 1.62
Basic weighted average shares outstanding (in shares)	109.5	111.5	109.9	111.5
Diluted weighted average shares outstanding (in shares)	109.6	111.6	110.1	111.6
Transaction fees
Revenues:
Total revenues	$ 618.3	$ 426.9	$ 1,279.8	$ 856.2
Access and capacity fees
Revenues:
Total revenues	55.7	54.5	113.4	108.9
Market data fees
Revenues:
Total revenues	58.7	51.8	114.9	103.4
Regulatory fees
Revenues:
Total revenues	128.7	79.7	265.5	138.4
Other revenue
Revenues:
Total revenues	7.3	7.7	16.6	15.2
Liquidity payments
Cost of revenues:
Total cost of revenues	415.6	235.8	808.0	479.5
Routing and clearing
Cost of revenues:
Total cost of revenues	17.7	9.2	33.7	18.4
Section 31 fees
Cost of revenues:
Total cost of revenues	119.0	70.3	246.4	118.5
Royalty fees
Cost of revenues:
Total cost of revenues	19.4	21.9	46.8	42.9
Other
Cost of revenues:
Total cost of revenues	$ 0.1	$ 0.2	$ 0.1	$ 0.2