GOODWILL AND INTANGIBLE ASSETS, NET - Schedule of intangible assets by segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Finite-lived Intangible Assets
Balance at beginning of the year			$ 1,902.6
Amortization	$ (39.7)	$ (42.7)	(81.8)	$ (57.2)
Changes in foreign currency exchange rates			(7.8)
Balance at end of the year	1,813.0		1,813.0
Options
Finite-lived Intangible Assets
Balance at beginning of the year			198.7
Amortization			(8.5)
Balance at end of the year	190.2		190.2
U.S. Equities
Finite-lived Intangible Assets
Balance at beginning of the year			1,071.8
Amortization			(41.5)
Balance at end of the year	1,030.3		1,030.3
European Equities
Finite-lived Intangible Assets
Balance at beginning of the year			427.0
Amortization			(14.7)
Changes in foreign currency exchange rates			(7.8)
Balance at end of the year	404.5		404.5
Global FX
Finite-lived Intangible Assets
Balance at beginning of the year			199.6
Amortization			(16.5)
Balance at end of the year	183.1		183.1
Corporate and Other
Finite-lived Intangible Assets
Balance at beginning of the year			5.5
Amortization			(0.6)
Balance at end of the year	4.9		4.9
Futures
Finite-lived Intangible Assets
Balance at end of the year	$ 0.0		$ 0.0