FAIR VALUE MEASUREMENTS - FAIR VALUE ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS, UNOBSERVABLE INPUT RECONCILIATION (Details) - Fair Value, Inputs, Level 3 - Fair value, Measurements, Recurring - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021
Corporate securities
Assets:
Beginning balance	$ 89.7	$ 146.9
Purchases, sales, issuances and settlements, net	15.2	25.0
Total realized and unrealized gains (losses) included in net income	(10.4)	(0.5)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(35.5)	1.4
Transfers into level 3	76.2	0.0
Transfers out of level 3	(7.4)	(83.1)
Ending balance	127.8	89.7
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(9.8)	(0.5)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(37.8)	(0.6)
Corporate securities | Investments held by variable interest entities - corporate securities
Assets:
Beginning balance	2.2	0.0
Purchases, sales, issuances and settlements, net	(2.1)	(0.2)
Total realized and unrealized gains (losses) included in net income	(0.1)	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.1
Transfers into level 3	0.0	2.3
Transfers out of level 3	0.0	0.0
Ending balance	0.0	2.2
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	0.0	0.1
Asset-backed securities
Assets:
Beginning balance	26.6	14.3
Purchases, sales, issuances and settlements, net	38.1	14.4
Total realized and unrealized gains (losses) included in net income	(0.1)	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(10.3)	(0.1)
Transfers into level 3	2.7	0.0
Transfers out of level 3	0.0	(2.0)
Ending balance	57.0	26.6
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(10.3)	(0.1)
Non-agency residential mortgage-backed securities
Assets:
Beginning balance	0.0	1.6
Purchases, sales, issuances and settlements, net	14.3	0.0
Total realized and unrealized gains (losses) included in net income	(0.3)	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(24.7)	0.0
Transfers into level 3	66.9	0.0
Transfers out of level 3	0.0	(1.6)
Ending balance	56.2	0.0
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(24.7)	0.0
Collateralized loan obligations
Assets:
Beginning balance	5.0	0.0
Purchases, sales, issuances and settlements, net	0.0	5.0
Total realized and unrealized gains (losses) included in net income	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(0.2)	0.0
Transfers into level 3	3.6	0.0
Transfers out of level 3	(5.0)	0.0
Ending balance	3.4	5.0
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(0.4)	0.0
Commercial mortgage-backed securities
Assets:
Beginning balance	19.0	0.0
Purchases, sales, issuances and settlements, net	0.0	6.5
Total realized and unrealized gains (losses) included in net income	0.0	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(4.5)	(0.7)
Transfers into level 3	0.0	13.2
Transfers out of level 3	0.0	0.0
Ending balance	14.5	19.0
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(4.6)	(0.8)
Total fixed maturities, available for sale
Assets:
Beginning balance	140.3	162.8
Purchases, sales, issuances and settlements, net	67.6	50.9
Total realized and unrealized gains (losses) included in net income	(10.8)	(0.5)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(75.2)	0.6
Transfers into level 3	149.4	13.2
Transfers out of level 3	(12.4)	(86.7)
Ending balance	258.9	140.3
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(9.8)	(0.5)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	(77.8)	(1.5)
Equity securities - corporate securities
Assets:
Beginning balance	11.5	26.8
Purchases, sales, issuances and settlements, net	63.9	(8.0)
Total realized and unrealized gains (losses) included in net income	0.3	(7.3)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.0
Transfers into level 3	0.0	0.0
Transfers out of level 3	0.0	0.0
Ending balance	75.7	11.5
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.4	(7.3)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	0.0	0.0
Trading securities - non-agency residential mortgage-backed securities
Assets:
Beginning balance	3.5	5.9
Purchases, sales, issuances and settlements, net	0.0	(2.5)
Total realized and unrealized gains (losses) included in net income	(0.3)	(0.4)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.5
Transfers into level 3	0.8	0.0
Transfers out of level 3	(3.5)	0.0
Ending balance	0.5	3.5
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(0.3)	(0.4)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	0.0	0.0
Trading securities - commercial mortgage-backed securities
Assets:
Beginning balance	12.9	17.0
Purchases, sales, issuances and settlements, net	0.0	0.0
Total realized and unrealized gains (losses) included in net income	0.0	(0.1)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	0.7
Transfers into level 3	0.0	0.0
Transfers out of level 3	(12.9)	(4.7)
Ending balance	0.0	12.9
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0	(0.1)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	0.0	0.0
Total trading securities
Assets:
Beginning balance	16.4	22.9
Purchases, sales, issuances and settlements, net	0.0	(2.5)
Total realized and unrealized gains (losses) included in net income	(0.3)	(0.5)
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	1.2
Transfers into level 3	0.8	0.0
Transfers out of level 3	(16.4)	(4.7)
Ending balance	0.5	16.4
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(0.3)	(0.5)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	0.0	0.0
Other invested assets - residual tranches
Assets:
Beginning balance	0.0
Purchases, sales, issuances and settlements, net	18.6
Total realized and unrealized gains (losses) included in net income	0.0
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(2.1)
Transfers into level 3	1.8
Transfers out of level 3	0.0
Ending balance	18.3	$ 0.0
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets still held as of the reporting date	$ (2.1)