FAIR VALUE OF ASSETS AND LIABILITIES - Tri-Level Hierarchy, Carrying Value (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	$ 393.9	$ 668.7
Derivative Asset Statement Of Financial Position Extensible Enumeration Not Disclosed Flag	Total return swap related to deferred compensation plans
Total return swap related to deferred compensation plans	$ 4.9
Total	1,599.0	1,754.5
Total	(1.3)	(2.9)
Equity method investments	631.8	621.2
Other investments	14.9	24.7
Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swap related to deferred compensation plans		(1.6)
Contingent consideration liability	(1.3)	(1.3)
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	927.8	760.8
Seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	75.7	177.9
Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	196.7	146.1
Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	0.0	1.0
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	393.9	668.7
Total return swap related to deferred compensation plans	0.0
Total	1,594.1	1,754.5
Total	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swap related to deferred compensation plans		0.0
Contingent consideration liability	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	927.8	760.8
Quoted Prices in Active Markets for Identical Assets (Level 1) | Seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	75.7	177.9
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	196.7	146.1
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		1.0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	0.0	0.0
Total return swap related to deferred compensation plans	4.9
Total	4.9	0.0
Total	0.0	(1.6)
Significant Other Observable Inputs (Level 2) | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swap related to deferred compensation plans		(1.6)
Contingent consideration liability	0.0	0.0
Significant Other Observable Inputs (Level 2) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Significant Other Observable Inputs (Level 2) | Seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	0.0	0.0
Significant Other Observable Inputs (Level 2) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	0.0	0.0
Significant Other Observable Inputs (Level 2) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		0.0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	0.0	0.0
Total return swap related to deferred compensation plans	0.0
Total	0.0	0.0
Total	(1.3)	(1.3)
Significant Unobservable Inputs (Level 3) | Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swap related to deferred compensation plans		0.0
Contingent consideration liability	(1.3)	(1.3)
Significant Unobservable Inputs (Level 3) | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	0.0	0.0
Significant Unobservable Inputs (Level 3) | Seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	0.0	0.0
Significant Unobservable Inputs (Level 3) | Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	$ 0.0	0.0
Significant Unobservable Inputs (Level 3) | Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments		$ 0.0