INVESTMENTS - NET REALIZED INVESTMENT GAINS (LOSSES) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Gain (Loss) on Securities [Line Items]
Total realized gains (losses)	$ 16.3	$ 2.3	$ (55.4)	$ 23.7
Change in allowance for credit losses of other investments	5.0	0.0	(17.8)	0.0
Loss on dissolution of variable interest entity	0.0	0.0	0.0	(5.1)
Increase (decrease) in estimated fair value of equity securities			(6.1)
Variable interest entities, change in allowance for current expected credit losses	5.6		(12.3)
Decrease in estimated fair value of trading securities			(1.2)
Total fixed maturities, available for sale
Gain (Loss) on Securities [Line Items]
Gross realized gains on sale	2.8	3.2	41.6	70.0
Gross realized losses on sale	(0.9)	(1.1)	(51.3)	(53.4)
Change in allowance for credit losses and other-than-temporary impairment losses	3.1	(3.4)	(13.6)	(5.6)
Total realized gains (losses)	5.0	(1.3)	(23.3)	11.0
Equity securities - corporate securities
Gain (Loss) on Securities [Line Items]
Total realized gains (losses)	1.4	0.6	(8.8)	11.4
Other
Gain (Loss) on Securities [Line Items]
Total realized gains (losses)	$ 4.9	$ 3.0	$ (5.5)	$ 6.4