FAIR VALUE MEASUREMENTS - FAIR VALUE INPUTS (Details) $ in Millions	Sep. 30, 2020 USD ($)	Jan. 01, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 22,702.9	$ 21,293.1	$ 21,295.2	$ 18,443.7
Equity securities - corporate securities	62.1		44.1
Corporate securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	13,970.6		12,935.3
Asset-backed securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	1,117.4		1,387.9
Significant unobservable inputs  (Level 3)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Total assets carried at fair value by category	163.5		213.3
Significant unobservable inputs  (Level 3) | Unadjusted third-party price source
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other assets categorized as Level 3	137.0		57.2
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Embedded derivatives related to fixed index annuity products	$ 1,598.9		$ 1,565.4
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Projected portfolio yields | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0365		0.0471
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Projected portfolio yields | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0425		0.0498
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Projected portfolio yields | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0423		0.0472
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Discount rates | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0000		0.0124
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Discount rates | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0246		0.0307
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Discount rates | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0083		0.0188
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Surrender rates | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.0130		0.0160
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Surrender rates | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.2400		0.3190
Significant unobservable inputs  (Level 3) | Discounted projected embedded derivatives | Surrender rates | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, liabilities	0.1000		0.1090
Significant unobservable inputs  (Level 3) | Corporate securities | Discounted cash flow analysis
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 5.6		$ 134.2
Significant unobservable inputs  (Level 3) | Corporate securities | Discounted cash flow analysis | Discount margins | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0433		0.0107
Significant unobservable inputs  (Level 3) | Corporate securities | Discounted cash flow analysis | Discount margins | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0457		0.0842
Significant unobservable inputs  (Level 3) | Corporate securities | Discounted cash flow analysis | Discount margins | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0455		0.0191
Significant unobservable inputs  (Level 3) | Corporate securities | Recovery method
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale			$ 1.0
Significant unobservable inputs  (Level 3) | Corporate securities | Recovery method | Percent of recovery expected | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets			0.1277
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discounted cash flow analysis
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fixed maturities, available for sale	$ 12.6		$ 12.6
Significant unobservable inputs  (Level 3) | Asset-backed securities | Discounted cash flow analysis | Discount margins | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.0247		0.0166
Significant unobservable inputs  (Level 3) | Equity Securities | Recovery method
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity securities - corporate securities	$ 8.3		$ 8.3
Significant unobservable inputs  (Level 3) | Equity Securities | Recovery method | Percent of recovery expected | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.5927		0.5927
Significant unobservable inputs  (Level 3) | Equity Securities | Recovery method | Percent of recovery expected | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	1.0000		1.0000
Significant unobservable inputs  (Level 3) | Equity Securities | Recovery method | Percent of recovery expected | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Unobservable inputs, assets	0.5952		0.5952