FAIR VALUE MEASUREMENTS - FAIR VALUE INPUTS (Details) $ in Millions	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)	Jan. 01, 2020 USD ($)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 20,353.4	$ 24,805.4	$ 21,293.1
Equity securities - corporate securities	135.3	131.1
Significant unobservable inputs  (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets carried at fair value by category	353.4	170.4
Significant unobservable inputs  (Level 3) | Unadjusted third-party price source
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other assets	249.0	132.4
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Embedded derivatives associated with fixed indexed annuity products (classified as policyholder account liabilities)	$ 1,297.0	$ 1,724.1
Significant unobservable inputs  (Level 3) | Discount margins/rate | Discounted projected embedded derivatives | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0377	0.0031
Significant unobservable inputs  (Level 3) | Discount margins/rate | Discounted projected embedded derivatives | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0548	0.0318
Significant unobservable inputs  (Level 3) | Discount margins/rate | Discounted projected embedded derivatives | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0447	0.0189
Significant unobservable inputs  (Level 3) | EBITDA Multiple | Market comparables | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	8.5
Significant unobservable inputs  (Level 3) | Percent of recovery expected | Recovery method | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0000	0.0000
Unobservable inputs, assets	0.0000	0.0000
Significant unobservable inputs  (Level 3) | Percent of recovery expected | Recovery method | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.3500	1.0000
Unobservable inputs, assets	1.0000	1.0000
Significant unobservable inputs  (Level 3) | Percent of recovery expected | Recovery method | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.3500	1.0000
Unobservable inputs, assets	1.0000	1.0000
Significant unobservable inputs  (Level 3) | Projected portfolio yields | Discounted projected embedded derivatives | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0430	0.0398
Significant unobservable inputs  (Level 3) | Projected portfolio yields | Discounted projected embedded derivatives | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0463	0.0437
Significant unobservable inputs  (Level 3) | Projected portfolio yields | Discounted projected embedded derivatives | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0431	0.0399
Significant unobservable inputs  (Level 3) | Surrender rates | Discounted projected embedded derivatives | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0190	0.0150
Significant unobservable inputs  (Level 3) | Surrender rates | Discounted projected embedded derivatives | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.2770	0.2640
Significant unobservable inputs  (Level 3) | Surrender rates | Discounted projected embedded derivatives | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0920	0.0900
Significant unobservable inputs  (Level 3) | Corporate securities | Discounted cash flow analysis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 2.9	$ 0.1
Significant unobservable inputs  (Level 3) | Corporate securities | Recovery method
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	3.5	2.3
Significant unobservable inputs  (Level 3) | Corporate securities | Unadjusted purchase price
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 0.5	$ 12.5
Significant unobservable inputs  (Level 3) | Corporate securities | Discount margins/rate | Discounted cash flow analysis | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0223
Significant unobservable inputs  (Level 3) | Corporate securities | Discount margins/rate | Discounted cash flow analysis | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0394
Significant unobservable inputs  (Level 3) | Corporate securities | Discount margins/rate | Discounted cash flow analysis | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0225	0.0449
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discounted cash flow analysis
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 21.8	$ 11.6
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discount margins/rate | Discounted cash flow analysis | Minimum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0250
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discount margins/rate | Discounted cash flow analysis | Maximum
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0386
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discount margins/rate | Discounted cash flow analysis | Weighted Average
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0330	0.0150
Significant unobservable inputs  (Level 3) | Equity Securities | Recovery method
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities - corporate securities	$ 0.1	$ 3.3
Significant unobservable inputs  (Level 3) | Equity Securities | Unadjusted purchase price
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities - corporate securities	11.7	$ 8.2
Significant unobservable inputs  (Level 3) | Equity Securities | Market comparables
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities - corporate securities	$ 63.9