INVESTMENTS (SCHEDULE OF FIXED MATURITIES FOR AVAILABLE FOR SALE AND EQUITY SECURITIES) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 19,803.1	$ 18,947.0
Gross unrealized gains	1,487.4	1,324.4
Gross unrealized losses	(194.3)	(388.5)
Estimated fair value	21,096.2	19,882.9
Other-than-temporary impairments included in accumulated other comprehensive income	(8.0)	(4.9)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	580.7	447.4
Gross unrealized gains	11.5	18.4
Gross unrealized losses	(8.0)	(2.8)
Estimated fair value	584.2	463.0
Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	16,733.1	16,712.5
Gross unrealized gains	1,359.2	1,241.4
Gross unrealized losses	(143.9)	(293.8)
Estimated fair value	17,948.4	17,660.1
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	11,582.6	11,477.5
Gross unrealized gains	1,073.9	929.4
Gross unrealized losses	(99.8)	(262.9)
Estimated fair value	12,556.7	12,144.0
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | US treasury and government [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	143.8	172.5
Gross unrealized gains	20.5	22.3
Gross unrealized losses	0.0	(0.3)
Estimated fair value	164.3	194.5
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,798.2	1,889.6
Gross unrealized gains	186.7	208.6
Gross unrealized losses	(7.9)	(3.7)
Estimated fair value	1,977.0	2,094.5
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Debt securities issued by foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	37.1	18.9
Gross unrealized gains	0.2	0.0
Gross unrealized losses	(0.4)	(0.6)
Estimated fair value	36.9	18.3
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,169.6	979.8
Gross unrealized gains	29.2	22.1
Gross unrealized losses	(8.7)	(7.2)
Estimated fair value	1,190.1	994.7
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	227.5	188.5
Gross unrealized gains	1.0	0.4
Gross unrealized losses	(0.3)	(2.2)
Estimated fair value	228.2	186.7
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,467.2	1,531.7
Gross unrealized gains	32.9	41.3
Gross unrealized losses	(26.6)	(16.3)
Estimated fair value	1,473.5	1,556.7
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Mortgage pass-through securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2.3	3.1
Gross unrealized gains	0.2	0.3
Gross unrealized losses	0.0	0.0
Estimated fair value	2.5	3.4
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Investment Grade [Member] | Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	304.8	450.9
Gross unrealized gains	14.6	17.0
Gross unrealized losses	(0.2)	(0.6)
Estimated fair value	319.2	467.3
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Non Investment Grade [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,070.0	2,234.5
Gross unrealized gains	128.2	83.0
Gross unrealized losses	(50.4)	(94.7)
Estimated fair value	3,147.8	2,222.8
Other-than-temporary impairments included in accumulated other comprehensive income	(8.0)	(4.9)
Non Investment Grade [Member] | Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	967.3	798.5
Gross unrealized gains	26.1	8.3
Gross unrealized losses	(39.2)	(82.3)
Estimated fair value	954.2	724.5
Other-than-temporary impairments included in accumulated other comprehensive income	(3.6)	0.0
Non Investment Grade [Member] | States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	13.6	13.6
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(1.7)	(3.9)
Estimated fair value	11.9	9.7
Other-than-temporary impairments included in accumulated other comprehensive income	(3.0)	(3.0)
Non Investment Grade [Member] | Debt securities issued by foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost		2.4
Gross unrealized gains		0.0
Gross unrealized losses		0.0
Estimated fair value		2.4
Other-than-temporary impairments included in accumulated other comprehensive income		0.0
Non Investment Grade [Member] | Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,471.9	838.0
Gross unrealized gains	55.1	25.7
Gross unrealized losses	(6.8)	(6.2)
Estimated fair value	1,520.2	857.5
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Non Investment Grade [Member] | Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2.5
Gross unrealized gains	0.0
Gross unrealized losses	0.0
Estimated fair value	2.5
Other-than-temporary impairments included in accumulated other comprehensive income	0.0
Non Investment Grade [Member] | Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	63.8	49.9
Gross unrealized gains	0.2	0.0
Gross unrealized losses	(1.3)	(1.3)
Estimated fair value	62.7	48.6
Other-than-temporary impairments included in accumulated other comprehensive income	0.0	0.0
Non Investment Grade [Member] | Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	550.9	532.1
Gross unrealized gains	46.8	49.0
Gross unrealized losses	(1.4)	(1.0)
Estimated fair value	596.3	580.1
Other-than-temporary impairments included in accumulated other comprehensive income	$ (1.4)	$ (1.9)