GOODWILL AND INTANGIBLE ASSETS, NET - Schedule of intangible assets by segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Finite-lived Intangible Assets
Balance at beginning of the year	$ 1,589.9
Additions	20.3
Amortization	(32.5)	$ (37.5)
Changes in foreign currency exchange rates	(22.2)
Balance at end of the year	1,555.5
Options
Finite-lived Intangible Assets
Balance at beginning of the year	166.6
Additions	20.3
Amortization	(3.9)
Balance at end of the year	183.0
U.S. Equities
Finite-lived Intangible Assets
Balance at beginning of the year	921.4
Amortization	(16.0)
Balance at end of the year	905.4
European Equities
Finite-lived Intangible Assets
Balance at beginning of the year	363.7
Amortization	(5.9)
Changes in foreign currency exchange rates	(22.2)
Balance at end of the year	335.6
Global FX
Finite-lived Intangible Assets
Balance at beginning of the year	138.2
Amortization	(6.7)
Balance at end of the year	131.5
Futures
Finite-lived Intangible Assets
Balance at end of the year	$ 0.0