OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE LOSS - Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning of period	$ 10,713.8	$ 10,134.0	$ 10,506.1	$ 9,697.1
Other comprehensive income (loss) before income taxes	(5.4)	17.6	15.4	14.3
Net deferred tax benefits (income taxes)	0.7	(0.5)	(2.0)	(0.1)
Other comprehensive income (loss)	(4.7)	17.1	13.4	14.2
Balances at end of period	10,984.5	10,452.7	10,984.5	10,452.7
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning of period	(44.6)	(49.2)	(51.7)	(47.5)
Other comprehensive income (loss) before reclassifications and income taxes	(2.6)	9.6	10.3	7.5
Reclassification adjustments recognized in non-operating income	0.0	0.6	(3.1)	0.6
Other comprehensive income (loss) before income taxes	(2.6)	10.2	7.2	8.1
Net deferred tax benefits (income taxes)	0.7	(0.5)	(2.0)
Other comprehensive income (loss)	(1.9)	9.7	5.2	8.0
Balances at end of period	(46.5)	(39.5)	(46.5)	(39.5)
Currency translation adjustments | Equity method investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning of period	(52.5)	(51.5)	(49.5)	(51.9)
Other comprehensive income (loss) before reclassifications and income taxes	(0.2)	0.1	(3.7)	0.6
Reclassification adjustments recognized in non-operating income	0.0	0.0	0.0	0.0
Other comprehensive income (loss) before income taxes	(0.2)	0.1	(3.7)	0.6
Net deferred tax benefits (income taxes)	0.1	1.8	0.6	1.7
Other comprehensive income (loss)	(0.1)	1.9	(3.1)	2.3
Balances at end of period	(52.6)	(49.6)	(52.6)	(49.6)
Currency translation adjustments | Consolidated investment products - variable interest entities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balances at beginning of period	7.9	2.3	(2.2)	4.4
Other comprehensive income (loss) before reclassifications and income taxes	(2.4)	9.5	14.0	6.9
Reclassification adjustments recognized in non-operating income	0.0	0.6	(3.1)	0.6
Other comprehensive income (loss) before income taxes	(2.4)	10.1	10.9	7.5
Net deferred tax benefits (income taxes)	0.6	(2.3)	(2.6)	(1.8)
Other comprehensive income (loss)	(1.8)	7.8	8.3	5.7
Balances at end of period	$ 6.1	$ 10.1	$ 6.1	$ 10.1