INVESTMENTS IN VARIABLE INTEREST ENTITIES - NARRATIVE (Details) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($) investment	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) investment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items]
Variable interest entity amortized cost securities held	$ 1,550.6		$ 1,550.6
Variable interest entity, gross unrealized gains fixed maturity securities	3.8		3.8
Variable interest entity gross unrealized losses fixed maturity securities	11.1		11.1
Estimated fair value of fixed maturity securities	1,543.3		1,543.3
Variable interest entities net realized gain (loss) on investments			(2.9)	$ (1.8)
Variable interest entities net gain (loss) from sale of fixed maturity investments				2.3
Loss on dissolution of a variable interest entity	$ 0.0	$ (3.7)	$ 0.0	(3.7)
Total other than temporary impairment losses on investments held by variable interest entities				(0.4)
Number of investments held by VIE, in default | investment	0		0
Variable interest entities, investments sold			$ 44.0	72.8
Variable interest entity, gross investment losses from sale			3.1	$ 1.8
Investments held in limited partnerships	$ 508.6		508.6
Unfunded commitments to limited partnerships	252.1		252.1
Less than twelve months
Variable Interest Entity [Line Items]
Fair value investments held by variable interest entity that had been in an unrealized loss position	937.0		937.0		$ 445.4
Gross unrealized losses for a period			8.7		4.9
Greater than twelve months
Variable Interest Entity [Line Items]
Fair value investments held by variable interest entity that had been in an unrealized loss position	$ 52.3		52.3		28.4
Gross unrealized losses for a period			$ 2.4		$ 1.7