FAIR VALUE MEASUREMENTS - FAIR VALUE INPUTS (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 24,936.3	$ 24,487.1
Other invested assets	433.4	415.1
Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	289.6	279.4
Other invested assets	155.6	148.3
Policyholder account balances	$ 1,096.0	$ 1,057.1
Level 3 [Member] | Interest sensitive products [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Weighted average projected portfolio yields	5.42%	5.42%
Weighted average discount rates	1.44%	1.94%
Weighted average surrender rates	14.09%	14.09%
Level 3 [Member] | Minimum [Member] | Interest sensitive products [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.15%	5.15%
Discount rates	0.00%	0.00%
Surrender rates	1.67%	1.67%
Level 3 [Member] | Maximum [Member] | Interest sensitive products [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Projected portfolio yields	5.61%	5.61%
Discount rates	2.80%	3.18%
Surrender rates	46.56%	46.56%
Level 3 [Member] | Corporate securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 78.8	$ 76.9
Weighted average discount rate	5.55%	5.35%
Level 3 [Member] | Corporate securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	1.80%	1.65%
Level 3 [Member] | Corporate securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	11.29%	9.74%
Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 22.8	$ 22.2
Weighted average discount rate	3.47%	3.50%
Level 3 [Member] | Asset-backed securities [Member] | Minimum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	2.89%	2.83%
Level 3 [Member] | Asset-backed securities [Member] | Maximum [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	4.25%	4.45%
Level 3 [Member] | Equity securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 32.4	$ 32.0