FAIR VALUE OF ASSETS AND LIABILITIES - Fair Value of Financial Instruments Held by Consolidated Investments (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022	Mar. 31, 2022
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	$ 889.0	$ 1,234.7	$ 1,309.6
Foreign time deposits	25.9	25.7
Assets held for policyholders	648.5	668.7
Policyholder payables	(648.5)	(668.7)
Total return swaps related to deferred compensation plans		(1.6)
Total return swaps related to deferred compensation plans	11.8
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	889.0	1,234.7
Equity investments	272.1	325.0
Foreign time deposits	25.9	25.7
Assets held for policyholders	648.5	668.7
Policyholder payables	(648.5)	(668.7)
Fair Value | Total return swaps related to deferred compensation plans
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total return swaps related to deferred compensation plans	$ 11.8
Total return swaps related to deferred compensation plans		$ (1.6)