INVESTMENTS - SUMMARY OF INVESTMENTS WITH UNREALIZED LOSSES BY INVESTMENT CATEGORY (DETAILS) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	$ 1,622.7	$ 3,543.8
Unrealized losses, less than 12 months	(54.0)	(178.1)
Fair value, twelve months or longer	300.7	285.8
Unrealized losses, 12 months or longer	(22.3)	(30.2)
Fair value, total	1,923.4	3,829.6
Unrealized losses, total	(76.3)	(208.3)
US treasury and government
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	19.2	23.8
Unrealized losses, less than 12 months	(0.4)	(0.6)
Fair value, twelve months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Fair value, total	19.2	23.8
Unrealized losses, total	(0.4)	(0.6)
US states and political subdivisions debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	186.2	473.6
Unrealized losses, less than 12 months	(11.6)	(30.3)
Fair value, twelve months or longer	63.5	79.2
Unrealized losses, 12 months or longer	(4.6)	(8.7)
Fair value, total	249.7	552.8
Unrealized losses, total	(16.2)	(39.0)
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	980.5	2,406.1
Unrealized losses, less than 12 months	(35.7)	(132.8)
Fair value, twelve months or longer	196.3	170.3
Unrealized losses, 12 months or longer	(16.5)	(20.8)
Fair value, total	1,176.8	2,576.4
Unrealized losses, total	(52.2)	(153.6)
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	229.7	308.4
Unrealized losses, less than 12 months	(2.9)	(6.5)
Fair value, twelve months or longer	35.3	32.5
Unrealized losses, 12 months or longer	(0.8)	(0.7)
Fair value, total	265.0	340.9
Unrealized losses, total	(3.7)	(7.2)
Collateralized debt obligations
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	26.9	46.7
Unrealized losses, less than 12 months	(0.6)	(0.5)
Fair value, twelve months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Fair value, total	26.9	46.7
Unrealized losses, total	(0.6)	(0.5)
Commercial mortgage backed securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	77.1	161.8
Unrealized losses, less than 12 months	(2.2)	(5.8)
Fair value, twelve months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Fair value, total	77.1	161.8
Unrealized losses, total	(2.2)	(5.8)
Mortgage pass through securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	0.9	1.6
Unrealized losses, less than 12 months	0	0
Fair value, twelve months or longer	0.4	1.6
Unrealized losses, 12 months or longer	0	0
Fair value, total	1.3	3.2
Unrealized losses, total	0	0
Collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	102.2	121.8
Unrealized losses, less than 12 months	(0.6)	(1.6)
Fair value, twelve months or longer	5.2	2.2
Unrealized losses, 12 months or longer	(0.4)	0
Fair value, total	107.4	124.0
Unrealized losses, total	(1.0)	(1.6)
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Fair value, less than twelve months	41.0	26.8
Unrealized losses, less than 12 months	(2.2)	(4.9)
Fair value, twelve months or longer	0	0
Unrealized losses, 12 months or longer	0	0
Fair value, total	41.0	26.8
Unrealized losses, total	$ (2.2)	$ (4.9)