FAIR VALUE MEASUREMENTS - FAIR VALUE INPUTS (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 24,690.2	$ 24,908.3
Other invested assets	435.4	443.6
Policyholder account balances	$ 10,689.3	$ 10,707.2
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	1.96%	1.78%
Weighted average surrender rates	14.16%	14.16%
Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Weighted average discount rate	4.09%	2.58%
Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Weighted average discount rate	2.96%	2.95%
Minimum [Member] | Level 3 [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	1.98%	1.98%
Minimum [Member] | Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	1.50%	1.48%
Minimum [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	1.92%	1.99%
Maximum [Member] | Level 3 [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	3.38%	2.74%
Surrender rates	47.88%	47.88%
Maximum [Member] | Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	6.05%	5.83%
Maximum [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Discount rate	4.15%	4.15%
Estimate of fair value measurement [Member] | Level 3 [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 258.2	$ 518.8
Other invested assets	146.6	148.1
Estimate of fair value measurement [Member] | Level 3 [Member] | Interest sensitive products [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Policyholder account balances	1,074.0	1,081.5
Estimate of fair value measurement [Member] | Level 3 [Member] | Corporate debt securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	52.3	312.1
Estimate of fair value measurement [Member] | Level 3 [Member] | Asset-backed securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	29.4	30.6
Estimate of fair value measurement [Member] | Level 3 [Member] | Equity securities [Member]
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Investments	$ 29.9	$ 28.0