FAIR VALUE OF ASSETS AND LIABILITIES - Tri-Level Hierarchy, Carrying Value (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	$ 444.0	$ 668.7
Total return swaps related to deferred compensation plans	19.3
Total	1,311.6	1,754.5
Total return swaps related to deferred compensation plans		(1.6)
Total	(1.3)	(2.9)
Foreign time deposits	15.1	25.7
Equity method investments	669.5	621.2
Other investments	24.6	24.7
Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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	565.4	760.8
Seed capital
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	96.2	177.9
Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	185.8	146.1
Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	0.9	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		668.7
Assets held for policyholders	444.0
Total return swaps related to deferred compensation plans	0.0
Total	1,292.3	1,754.5
Total return swaps related to deferred compensation plans		0.0
Total	0.0	0.0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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	565.4	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	96.2	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	185.8	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	0.9	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
Assets held for policyholders	0.0
Total return swaps related to deferred compensation plans	19.3
Total	19.3	0.0
Total return swaps related to deferred compensation plans		(1.6)
Total	0.0	(1.6)
Significant Other Observable Inputs (Level 2) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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	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
Assets held for policyholders	0.0
Total return swaps related to deferred compensation plans	0.0
Total	0.0	0.0
Total return swaps related to deferred compensation plans		0.0
Total	(1.3)	(1.3)
Significant Unobservable Inputs (Level 3) | Contingent consideration liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
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	$ 0.0