Condensed Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Total revenues	$ 675.4	$ 575.9	$ 1,897.5	$ 2,021.1
Cost of revenues:
Total cost of revenues	381.4	305.4	1,040.9	1,138.6
Revenues less cost of revenues	294.0	270.5	856.6	882.5
Operating expenses:
Compensation and benefits	49.7	51.8	150.0	168.1
Depreciation and amortization	42.9	50.3	133.8	154.9
Technology support services	10.6	10.6	34.3	34.5
Professional fees and outside services	17.5	16.6	52.9	51.9
Travel and promotional expenses	2.7	2.6	8.3	9.8
Facilities costs	2.7	3.3	7.8	8.6
Acquisition-related costs	16.7	5.9	39.8	23.3
Other expenses	3.8	3.3	11.7	8.5
Total operating expenses	146.6	144.4	438.6	459.6
Operating income	147.4	126.1	418.0	422.9
Non-operating (expenses) income:
Interest expense, net	(8.2)	(9.6)	(28.1)	(28.5)
Other (expense) income	1.7	(0.2)	(2.7)	1.1
Income before income tax provision	140.9	116.3	387.2	395.5
Income tax provision	35.0	30.6	102.7	108.7
Net income	105.9	85.7	284.5	286.8
Net loss attributable to redeemable noncontrolling interest	0.1	0.3	4.1	0.9
Net income excluding redeemable noncontrolling interest	106.0	86.0	288.6	287.7
Change in redemption value of redeemable noncontrolling interest	(0.1)	(0.3)	(0.5)	(0.9)
Net income allocated to participating securities	(0.4)	(0.7)	(1.5)	(2.1)
Net income allocated to common stockholders	$ 105.5	$ 85.0	$ 286.6	$ 284.7
Basic earnings per share (in dollars per share)	$ 0.95	$ 0.76	$ 2.57	$ 2.54
Diluted earnings per share (in dollars per share)	$ 0.94	$ 0.76	$ 2.56	$ 2.53
Basic weighted average shares outstanding (in shares)	111.6	111.4	111.6	112.0
Diluted weighted average shares outstanding (in shares)	111.9	111.8	112.0	112.4
Transaction fees
Revenues:
Total revenues	$ 465.8	$ 411.8	$ 1,322.0	$ 1,423.7
Access and capacity fees
Revenues:
Total revenues	55.7	53.6	164.6	156.4
Market data fees
Revenues:
Total revenues	56.3	47.6	159.7	154.3
Regulatory fees
Revenues:
Total revenues	88.1	55.2	226.5	260.8
Other revenue
Revenues:
Total revenues	9.5	7.7	24.7	25.9
Liquidity payments
Cost of revenues:
Total cost of revenues	269.7	229.2	749.2	803.2
Routing and clearing
Cost of revenues:
Total cost of revenues	9.3	8.7	27.7	28.8
Section 31 fees
Cost of revenues:
Total cost of revenues	79.4	46.8	197.9	236.7
Royalty fees
Cost of revenues:
Total cost of revenues	22.9	$ 20.7	65.8	$ 69.9
Other
Cost of revenues:
Total cost of revenues	$ 0.1		$ 0.3