FAIR VALUE OF ASSETS AND LIABILITIES - Fair Value of Financial Instruments Held by Consolidated Investments (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 895.7	$ 1,469.2	$ 889.0
Assets held for policyholders	0.0	393.9
Policyholder payables	0.0	(393.9)
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	895.7	1,469.2
Equity investments	297.2	272.4
Assets held for policyholders	0.0	393.9
Policyholder payables	0.0	(393.9)
Fair Value | Total return swaps related to deferred compensation plans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total return swap related to deferred compensation plans	$ 8.1	$ 4.9