INVESTMENTS - NET REALIZED INVESTMENT GAINS (LOSSES) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Gain (Loss) on Securities [Line Items]
Impairments of other investments	[1]	$ (3.4)	$ (2.1)	$ (5.6)	$ (2.1)
Total realized gains		2.3	394.6	23.7	390.4
Net realized investment gains (losses) before net realized gains on the transfer of assets related to reinsurance transaction		2.3	31.2	23.7	27.0
Net realized gains on the transfer of assets related to reinsurance transaction		0.0	363.4	0.0	363.4
VIEs
Gain (Loss) on Securities [Line Items]
Loss on dissolution of variable interest entity		0.0	0.0	(5.1)	0.0
Total fixed maturities, available for sale
Gain (Loss) on Securities [Line Items]
Gross realized gains on sale		3.2	5.4	70.0	45.5
Gross realized losses on sale		(1.1)	(11.4)	(53.4)	(36.9)
Impairment losses recognized		(3.4)	0.0	(5.6)	0.0
Total realized gains		(1.3)	(6.0)	11.0	8.6
Equity securities - corporate securities
Gain (Loss) on Securities [Line Items]
Total realized gains		0.6	(3.0)	11.4	(13.3)
Other
Gain (Loss) on Securities [Line Items]
Impairments of other investments		0.0	(2.1)	0.0	(2.1)
Total realized gains		$ 3.0	$ 42.3	6.4	33.8
Investments
Gain (Loss) on Securities [Line Items]
Increase (decrease) in estimated fair value of trading securities				$ 13.5	$ (6.0)

[1] No portion of the other-than-temporary impairments recognized in the periods was included in accumulated other comprehensive income.