INVESTMENTS - NET REALIZED INVESTMENT GAINS (LOSSES) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Gain (Loss) on Investments [Line Items]
Total other-than-temporary impairment losses	$ (4.2)	$ (13.6)	$ (12.6)	$ (23.6)
Other-than-temporary impairment losses recognized in accumulated other comprehensive income	(0.9)	0.0	(0.9)	0.0
Total realized gains	15.2	12.6	23.1	11.7
Loss on dissolution of variable interest entities	(3.7)	(7.3)	(3.7)	(7.3)
Investments
Gain (Loss) on Investments [Line Items]
Change in estimated fair value of trading securities			9.9
Total fixed maturities, available for sale
Gain (Loss) on Investments [Line Items]
Gross realized gains on sale	21.1	21.3	28.1	119.8
Gross realized losses on sale	(5.2)	(0.6)	(7.9)	(81.6)
Total other-than-temporary impairment losses	(3.6)	0.0	(6.8)	(6.3)
Other-than-temporary impairment losses recognized in accumulated other comprehensive income	(0.9)	0.0	(0.9)	0.0
Net impairment losses recognized	(4.5)	0.0	(7.7)	(6.3)
Total realized gains	11.4	20.7	12.5	31.9
Equity securities
Gain (Loss) on Investments [Line Items]
Total realized gains	0.0	4.0	1.9	4.1
Commercial mortgage loans
Gain (Loss) on Investments [Line Items]
Total realized gains	0.0	0.0	1.0	0.0
Impairments of other investments
Gain (Loss) on Investments [Line Items]
Total realized gains	(0.6)	(13.6)	(5.8)	(17.3)
Other
Gain (Loss) on Investments [Line Items]
Total realized gains	8.1	8.8	17.2	0.3
VIEs
Gain (Loss) on Investments [Line Items]
Loss on dissolution of variable interest entities	$ (3.7)	$ (7.3)	$ (3.7)	$ (7.3)