GOODWILL AND INTANGIBLE ASSETS, NET - Goodwill by segment (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Goodwill
Balance at beginning of the year	$ 2,707.4
Changes in foreign currency exchange rates	(6.1)
Balance at end of the year	2,701.3
Goodwill impairment loss	0.0
Options
Goodwill
Balance at beginning of the year	239.4
Balance at end of the year	239.4
U.S. Equities
Goodwill
Balance at beginning of the year	1,740.4
Balance at end of the year	1,740.4
European Equities
Goodwill
Balance at beginning of the year	441.6
Changes in foreign currency exchange rates	(6.1)
Balance at end of the year	435.5
Global FX
Goodwill
Balance at beginning of the year	267.2
Balance at end of the year	267.2
Corporate and Other
Goodwill
Balance at beginning of the year	18.8
Balance at end of the year	$ 18.8