Restructuring and other - Restructuring and Other Costs (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring and other	$ 27.8	$ 7.8	$ 1.1	$ 9.2	$ 36.3	$ 13.8	$ 4.2	$ 2.7	$ 45.9	$ 57.0	$ 21.9
NYX
Restructuring Cost and Reserve [Line Items]
Restructuring and other									15.1
Employee severance
Restructuring Cost and Reserve [Line Items]
Restructuring and other									9.8	36.2	16.9
Acquisition-related costs
Restructuring Cost and Reserve [Line Items]
Restructuring and other									21.1	2.5	1.5
Management restructuring and related costs
Restructuring Cost and Reserve [Line Items]
Restructuring and other									1.4	5.9	0.0
Restructuring, integration and other
Restructuring Cost and Reserve [Line Items]
Restructuring and other									$ 13.6	$ 12.4	$ 3.5