FAIR VALUE OF ASSETS AND LIABILITIES - Tri-Level Hierarchy, Carrying Value (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets held for policyholders	$ 2,232.2	$ 7,582.1
Total return swaps related to deferred compensation plans		5.1
Total	3,712.7	8,894.8
Total	(11.6)	(18.6)
Foreign time deposits	29.1	29.9
Equity method investments	545.1	426.1
Other investments	7.9	10.5
Contingent Consideration Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	(3.4)	(18.6)
Contingent Consideration Liability | Total Return Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swaps related to deferred compensation plans	(8.2)
Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Money market funds	1,156.2	947.3
Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	102.8	153.5
Investments related to deferred compensation plans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	218.3	202.7
Other equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	3.2	4.1
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	2,232.2	7,582.1
Total return swaps related to deferred compensation plans		0.0
Total	3,712.7	8,889.7
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) | Contingent Consideration Liability | Total Return Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swaps related to deferred compensation plans	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	1,156.2	947.3
Quoted Prices in Active Markets for Identical Assets (Level 1) | Seed money
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments	102.8	153.5
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	218.3	202.7
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	3.2	4.1
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 swaps related to deferred compensation plans		5.1
Total	0.0	5.1
Total	(8.2)	0.0
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
Significant Other Observable Inputs (Level 2) | Contingent Consideration Liability | Total Return Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swaps related to deferred compensation plans	(8.2)
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 money
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	0.0
Total return swaps related to deferred compensation plans		0.0
Total	0.0	0.0
Total	(3.4)	(18.6)
Significant Unobservable Inputs (Level 3) | Contingent Consideration Liability
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Contingent consideration liability	(3.4)	(18.6)
Significant Unobservable Inputs (Level 3) | Contingent Consideration Liability | Total Return Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total return swaps related to deferred compensation plans	0.0
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 money
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