OTHER COMPREHENSIVE INCOME AND ACCUMULATED OTHER COMPREHENSIVE LOSS - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balances	$ 10,506.1	$ 9,697.1	$ 9,030.2
Other comprehensive income (loss) before income taxes	12.4	(12.9)	20.1
Net deferred tax benefits (income taxes)	(1.4)	3.7	(1.9)
Total other comprehensive income (loss)	11.0	(9.2)	18.2
Ending balances	11,017.2	10,506.1	9,697.1
Consolidated investment products - variable interest entities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification adjustments recognized in non-operating income	(3.1)
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balances	(51.7)	(47.5)	(53.0)
Other comprehensive income (loss) before income taxes	5.7	(8.3)	7.4
Reclassification adjustments recognized in non-operating income	(3.1)	0.4
Other comprehensive income (loss) before income taxes	2.6	(7.9)
Net deferred tax benefits (income taxes)	(1.4)	3.7	(1.9)
Total other comprehensive income (loss)	1.2	(4.2)	5.5
Ending balances	(50.5)	(51.7)	(47.5)
Currency translation adjustments | Equity method investments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balances	(49.5)	(51.9)	(50.5)
Other comprehensive income (loss) before income taxes	(9.3)	0.7	(1.6)
Reclassification adjustments recognized in non-operating income	0.0	0.0
Other comprehensive income (loss) before income taxes	(9.3)	0.7
Net deferred tax benefits (income taxes)	1.4	1.7	0.2
Total other comprehensive income (loss)	(7.9)	2.4	(1.4)
Ending balances	(57.4)	(49.5)	(51.9)
Currency translation adjustments | Consolidated investment products - variable interest entities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balances	(2.2)	4.4	(2.5)
Other comprehensive income (loss) before income taxes	15.0	(9.0)	9.0
Reclassification adjustments recognized in non-operating income		0.4
Other comprehensive income (loss) before income taxes	11.9	(8.6)
Net deferred tax benefits (income taxes)	(2.8)	2.0	(2.1)
Total other comprehensive income (loss)	9.1	(6.6)	6.9
Ending balances	$ 6.9	$ (2.2)	$ 4.4