Condensed Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Revenues:
Total revenues	$ 620.6	$ 667.5	$ 1,222.1	$ 1,445.2
Cost of revenues:
Total cost of revenues	337.4	384.0	659.5	833.2
Revenues less cost of revenues	283.2	283.5	562.6	612.0
Operating expenses:
Compensation and benefits	52.2	57.4	100.3	116.3
Depreciation and amortization	43.7	50.4	90.9	104.6
Technology support services	11.8	11.8	23.7	23.9
Professional fees and outside services	19.2	17.3	35.4	35.3
Travel and promotional expenses	3.0	3.5	5.6	7.2
Facilities costs	3.0	2.9	5.1	5.3
Acquisition-related costs	20.8	8.6	23.1	17.4
Other expenses	4.3	2.5	7.9	5.2
Total operating expenses	158.0	154.4	292.0	315.2
Operating income	125.2	129.1	270.6	296.8
Non-operating (expenses) income:
Interest expense, net	(10.0)	(9.3)	(19.9)	(18.9)
Other (expense) income	4.4		(4.4)	1.3
Income before income tax provision	119.6	119.8	246.3	279.2
Income tax provision	35.1	36.8	67.7	78.1
Net income	84.5	83.0	178.6	201.1
Net loss attributable to redeemable noncontrolling interest	3.8	0.3	4.0	0.6
Net income excluding redeemable noncontrolling interest	88.3	83.3	182.6	201.7
Change in redemption value of redeemable noncontrolling interest	(0.2)	(0.3)	(0.4)	(0.6)
Net income allocated to participating securities	(0.5)	(0.6)	(1.1)	(1.4)
Net income allocated to common stockholders	$ 87.6	$ 82.4	$ 181.1	$ 199.7
Basic earnings per share (in dollars per share)	$ 0.79	$ 0.74	$ 1.62	$ 1.78
Diluted earnings per share (in dollars per share)	$ 0.78	$ 0.73	$ 1.62	$ 1.77
Basic weighted average shares outstanding (in shares)	111.5	112.0	111.5	112.2
Diluted weighted average shares outstanding (in shares)	111.6	112.3	111.6	112.6
Transaction fees
Revenues:
Total revenues	$ 426.9	$ 464.8	$ 856.2	$ 1,011.9
Access and capacity fees
Revenues:
Total revenues	54.5	52.2	108.9	102.8
Market data fees
Revenues:
Total revenues	51.8	52.5	103.4	106.7
Regulatory fees
Revenues:
Total revenues	79.7	89.3	138.4	205.6
Other revenue
Revenues:
Total revenues	7.7	8.7	15.2	18.2
Liquidity payments
Cost of revenues:
Total cost of revenues	235.8	271.1	479.5	574.0
Routing and clearing
Cost of revenues:
Total cost of revenues	9.2	9.8	18.4	20.1
Section 31 fees
Cost of revenues:
Total cost of revenues	70.3	81.1	118.5	189.9
Royalty fees
Cost of revenues:
Total cost of revenues	21.9	$ 22.0	42.9	$ 49.2
Other
Cost of revenues:
Total cost of revenues	$ 0.2		$ 0.2