Condensed Consolidated Statements of Income - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Total revenues	$ 667.5	$ 640.8	$ 1,445.2	$ 997.0
Cost of revenues:
Total cost of revenues	384.0	373.9	833.2	536.7
Revenues less cost of revenues	283.5	266.9	612.0	460.3
Operating expenses:
Compensation and benefits	57.4	50.0	116.3	97.8
Depreciation and amortization	50.4	55.8	104.6	80.9
Technology support services	11.8	12.0	23.9	19.5
Professional fees and outside services	17.3	16.9	35.3	31.3
Travel and promotional expenses	3.5	4.2	7.2	7.5
Facilities costs	2.9	2.7	5.3	4.8
Acquisition-related costs	8.6	4.7	17.4	69.9
Other expenses	2.5	2.8	5.2	4.7
Total operating expenses	154.4	149.1	315.2	316.4
Operating income	129.1	117.8	296.8	143.9
Non-operating (expenses) income:
Interest expense, net	(9.3)	(12.5)	(18.9)	(20.4)
Other income		0.8	1.3	0.9
Income before income tax provision	119.8	106.1	279.2	124.4
Income tax provision	36.8	38.1	78.1	41.2
Net income	83.0	68.0	201.1	83.2
Net loss attributable to redeemable noncontrolling interest	0.3	0.3	0.6	0.6
Net income excluding redeemable noncontrolling interest	83.3	68.3	201.7	83.8
Change in redemption value of redeemable noncontrolling interest	(0.3)	(0.3)	(0.6)	(0.6)
Net income allocated to participating securities	(0.6)	(0.7)	(1.4)	(0.8)
Net income allocated to common stockholders	$ 82.4	$ 67.3	$ 199.7	$ 82.4
Basic earnings per share (in dollars per share)	$ 0.74	$ 0.60	$ 1.78	$ 0.81
Diluted earnings per share (in dollars per share)	$ 0.73	$ 0.60	$ 1.77	$ 0.81
Basic weighted average shares outstanding (in shares)	112.0	112.1	112.2	102.1
Diluted weighted average shares outstanding (in shares)	112.3	112.5	112.6	102.3
Transaction fees
Revenues:
Total revenues	$ 464.8	$ 453.9	$ 1,011.9	$ 710.3
Access fees
Revenues:
Total revenues	31.2	29.7	59.8	47.5
Exchange services and other fees
Revenues:
Total revenues	21.0	19.9	43.0	35.3
Market data fees
Revenues:
Total revenues	52.5	48.0	106.7	70.5
Regulatory fees
Revenues:
Total revenues	89.3	83.3	205.6	121.6
Other revenue
Revenues:
Total revenues	8.7	6.0	18.2	11.8
Liquidity payments
Cost of revenues:
Total cost of revenues	271.1	266.5	574.0	371.8
Routing and clearing
Cost of revenues:
Total cost of revenues	9.8	12.2	20.1	18.5
Section 31 fees
Cost of revenues:
Total cost of revenues	81.1	74.6	189.9	104.6
Royalty fees
Cost of revenues:
Total cost of revenues	$ 22.0	$ 20.6	$ 49.2	$ 41.8