INVESTMENTS (SCHEDULE OF STRUCTURED SECURITIES) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	$ 6,106.3
Amortized cost	5,260.5
Estimated fair value	$ 5,395.2
Percent of fixed maturities	25.60%
Pass-throughs, sequential and equivalent securities[Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 664.8
Estimated fair value	$ 710.6
Percent of fixed maturities	3.40%
Planned amortization classes, target amortization classes and accretion-directed bonds [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 142.5
Estimated fair value	$ 156.4
Percent of fixed maturities	0.70%
Commercial Mortgage Backed Securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 1,531.0
Estimated fair value	$ 1,536.2
Percent of fixed maturities	7.30%
Asset-backed securities [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 2,641.5
Estimated fair value	$ 2,710.3
Percent of fixed maturities	12.90%
Collateralized debt obligations [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 230.0
Estimated fair value	$ 230.7
Percent of fixed maturities	1.10%
Other [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Amortized cost	$ 50.7
Estimated fair value	$ 51.0
Percent of fixed maturities	0.20%
Below 4 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	$ 1,993.9
Amortized cost	1,523.5
Estimated fair value	1,535.4
4 Percent - 5 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	1,757.6
Amortized cost	1,592.5
Estimated fair value	1,624.7
5 Percent - 6 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	1,772.2
Amortized cost	1,602.2
Estimated fair value	1,666.7
6 Percent - 7 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	388.1
Amortized cost	348.0
Estimated fair value	364.5
7 Percent - 8 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	55.8
Amortized cost	56.2
Estimated fair value	65.6
8 Percent and Above [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Par value	138.7
Amortized cost	138.1
Estimated fair value	$ 138.3
Minimum [Member] | 4 Percent - 5 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	4.00%
Minimum [Member] | 5 Percent - 6 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	5.00%
Minimum [Member] | 6 Percent - 7 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	6.00%
Minimum [Member] | 7 Percent - 8 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	7.00%
Minimum [Member] | 8 Percent and Above [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	8.00%
Maximum [Member] | Below 4 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	4.00%
Maximum [Member] | 4 Percent - 5 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	5.00%
Maximum [Member] | 5 Percent - 6 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	6.00%
Maximum [Member] | 6 Percent - 7 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	7.00%
Maximum [Member] | 7 Percent - 8 Percent [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Structured settlement, interest rate	8.00%