INVESTMENTS (SCHEDULE OF UNREALIZED LOSS ON INVESTMENTS) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	$ 3,440.8	$ 4,815.0
Unrealized losses, less than 12 months	(109.5)	(291.3)
Fair value, twelve months or greater, Fair value	1,059.8	495.0
Unrealized losses, 12 months or greater	(84.8)	(97.2)
Fair value, total	4,500.6	5,310.0
Unrealized losses, total	(194.3)	(388.5)
US treasury and government [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	8.0	43.6
Unrealized losses, less than 12 months	0.0	(0.3)
Fair value, twelve months or greater, Fair value	0.0	0.0
Unrealized losses, 12 months or greater	0.0	0.0
Fair value, total	8.0	43.6
Unrealized losses, total	0.0	(0.3)
States and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	176.3	156.8
Unrealized losses, less than 12 months	(7.8)	(4.1)
Fair value, twelve months or greater, Fair value	18.3	14.8
Unrealized losses, 12 months or greater	(1.8)	(3.5)
Fair value, total	194.6	171.6
Unrealized losses, total	(9.6)	(7.6)
Debt securities issued by foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	18.9	20.7
Unrealized losses, less than 12 months	(0.4)	(0.6)
Fair value, twelve months or greater, Fair value	0.0	0.0
Unrealized losses, 12 months or greater	0.0	0.0
Fair value, total	18.9	20.7
Unrealized losses, total	(0.4)	(0.6)
Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	1,907.6	2,913.6
Unrealized losses, less than 12 months	(75.5)	(255.7)
Fair value, twelve months or greater, Fair value	559.6	278.9
Unrealized losses, 12 months or greater	(63.5)	(89.5)
Fair value, total	2,467.2	3,192.5
Unrealized losses, total	(139.0)	(345.2)
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	692.9	930.3
Unrealized losses, less than 12 months	(8.5)	(11.7)
Fair value, twelve months or greater, Fair value	262.5	98.4
Unrealized losses, 12 months or greater	(7.0)	(1.7)
Fair value, total	955.4	1,028.7
Unrealized losses, total	(15.5)	(13.4)
Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	38.3	96.2
Unrealized losses, less than 12 months	(0.1)	(1.8)
Fair value, twelve months or greater, Fair value	30.8	36.3
Unrealized losses, 12 months or greater	(0.2)	(0.4)
Fair value, total	69.1	132.5
Unrealized losses, total	(0.3)	(2.2)
Commercial Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	525.2	556.0
Unrealized losses, less than 12 months	(16.6)	(16.1)
Fair value, twelve months or greater, Fair value	154.0	25.7
Unrealized losses, 12 months or greater	(11.3)	(1.5)
Fair value, total	679.2	581.7
Unrealized losses, total	(27.9)	(17.6)
Mortgage pass-through securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months		0.0
Unrealized losses, less than 12 months		0.0
Fair value, twelve months or greater, Fair value		0.1
Unrealized losses, 12 months or greater		0.0
Fair value, total		0.1
Unrealized losses, total		0.0
Collateralized mortgage obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	73.6	97.8
Unrealized losses, less than 12 months	(0.6)	(1.0)
Fair value, twelve months or greater, Fair value	34.6	40.8
Unrealized losses, 12 months or greater	(1.0)	(0.6)
Fair value, total	108.2	138.6
Unrealized losses, total	(1.6)	(1.6)
Equity securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	239.4	140.1
Unrealized losses, less than 12 months	(8.0)	(2.4)
Fair value, twelve months or greater, Fair value	0.0	2.4
Unrealized losses, 12 months or greater	0.0	(0.4)
Fair value, total	239.4	142.5
Unrealized losses, total	$ (8.0)	$ (2.8)