FAIR VALUE MEASUREMENTS - FAIR VALUE INPUTS (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 23,355.4	$ 27,854.1
Other invested assets	920.0	924.5
Significant unobservable inputs (Level 3)
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	175.4	284.6
Other invested assets	50.5	87.2
Policyholder account balances	$ 1,370.0	$ 1,334.8
Significant unobservable inputs (Level 3) | Minimum | Interest sensitive products
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.15%	5.15%
Discount rates	1.50%	0.92%
Surrender rates	1.20%	1.20%
Significant unobservable inputs (Level 3) | Maximum | Interest sensitive products
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.61%	5.61%
Discount rates	2.98%	2.51%
Surrender rates	46.40%	46.40%
Significant unobservable inputs (Level 3) | Weighted Average | Interest sensitive products
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.60%	5.60%
Discount rates	2.70%	2.00%
Surrender rates	12.30%	12.30%
Significant unobservable inputs (Level 3) | Corporate securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 98.0	$ 149.2
Significant unobservable inputs (Level 3) | Corporate securities | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins	1.35%	1.45%
Significant unobservable inputs (Level 3) | Corporate securities | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins	8.91%	71.29%
Significant unobservable inputs (Level 3) | Corporate securities | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins	3.94%	6.96%
Significant unobservable inputs (Level 3) | Asset-backed securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 12.1	$ 24.2
Significant unobservable inputs (Level 3) | Asset-backed securities | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins		1.80%
Significant unobservable inputs (Level 3) | Asset-backed securities | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins		3.71%
Significant unobservable inputs (Level 3) | Asset-backed securities | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount margins	1.87%	2.67%
Significant unobservable inputs (Level 3) | Equity securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 1.2	$ 1.1
Significant unobservable inputs (Level 3) | Equity securities | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
EBITDA multiple	110.00%	110.00%
Percent of recovery expected		59.10%
Recovery method | Significant unobservable inputs (Level 3) | Corporate securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 5.3	$ 2.8
Recovery method | Significant unobservable inputs (Level 3) | Corporate securities | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	0.00%	0.00%
Recovery method | Significant unobservable inputs (Level 3) | Corporate securities | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	62.20%	21.73%
Recovery method | Significant unobservable inputs (Level 3) | Corporate securities | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	57.70%	18.42%
Recovery method | Significant unobservable inputs (Level 3) | Equity securities
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 8.3	$ 20.1
Recovery method | Significant unobservable inputs (Level 3) | Equity securities | Minimum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	59.27%
Recovery method | Significant unobservable inputs (Level 3) | Equity securities | Maximum
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	100.00%
Recovery method | Significant unobservable inputs (Level 3) | Equity securities | Weighted Average
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percent of recovery expected	59.52%