FAIR VALUE MEASUREMENTS (FAIR VALUE INPUTS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 26,237.6	$ 24,487.1
Other invested assets	589.5	415.1
Policyholder account balances	10,912.7	10,762.3
Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	385.4	279.4
Income Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 148.5	$ 76.9
Weighted average discount rate	13.52%	5.35%
Income Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	1.35%	1.65%
Income Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	27.71%	9.74%
Income Approach Valuation Technique [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 24.0	$ 22.2
Weighted average discount rate	2.76%	3.50%
Income Approach Valuation Technique [Member] | Level 3 [Member] | Asset-backed securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	2.06%	2.83%
Income Approach Valuation Technique [Member] | Level 3 [Member] | Asset-backed securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	3.64%	4.45%
Cost Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 14.8
Weighted average discount rate	55.00%
Cost Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	5.00%
Cost Approach Valuation Technique [Member] | Level 3 [Member] | Corporate securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	69.00%
Market Approach Valuation Technique [Member] | Level 3 [Member] | Equity securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 25.2	$ 32.0
Weighted average projected cash flows	5.90%
Market Approach Valuation Technique [Member] | Level 3 [Member] | Equity securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected cash flows	0.40%
Market Approach Valuation Technique [Member] | Level 3 [Member] | Equity securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected cash flows	6.20%
Unadjusted Third-Party Price Source [Member] | Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Other invested assets	$ 172.9	148.3
Discounted Projected Embedded Derivatives [Member] | Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Policyholder account balances	$ 1,092.3	$ 1,057.1
Discounted Projected Embedded Derivatives [Member] | Level 3 [Member] | Interest sensitive products [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Weighted average projected portfolio yields	5.59%	5.42%
Weighted average discount rates	2.07%	1.94%
Weighted average surrender rates	13.52%	14.09%
Discounted Projected Embedded Derivatives [Member] | Level 3 [Member] | Interest sensitive products [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.15%	5.15%
Discount rates	0.18%	0.00%
Surrender rates	0.94%	1.67%
Discounted Projected Embedded Derivatives [Member] | Level 3 [Member] | Interest sensitive products [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.61%	5.61%
Discount rates	3.06%	3.18%
Surrender rates	46.48%	46.56%