FAIR VALUE MEASUREMENTS - FAIR VALUE ACTIVITY (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Embedded derivatives associated with fixed index annuity products (classified as policyholder account liabilities)
Liabilities:
Purchases	$ (43.8)	$ (39.8)	$ (133.4)	$ (115.5)
Sales	0.0	2.6	119.3	4.5
Issuances	(52.2)	(6.4)	(101.2)	(66.6)
Settlements	18.9	21.4	65.5	67.8
Purchases, sales, issuances and settlements, net	(77.1)	(22.2)	(49.8)	(109.8)
Corporate securities
Assets:
Purchases	1.3	0.0	11.0	0.1
Sales	(0.1)	(1.7)	(2.4)	(27.9)
Issuances	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Purchases, sales, issuances and settlements, net	1.2	(1.7)	8.6	(27.8)
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	120.6	171.5	120.6	171.5	$ 114.2	$ 178.8	$ 135.9	$ 158.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(1.0)	(1.8)	(1.1)	(4.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	1.8	2.3	4.6	10.6
Transfers into level 3	53.0	36.8	79.3	34.7
Transfers out of level 3	(48.6)	0.0	(149.6)	0.0
Amount of total gains (losses) included in our net income relating to assets and liabilities still held as of the reporting date	(1.0)	(1.8)	(1.2)	(4.0)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets and liabilities still held as of the reporting date	1.1		2.3
Corporate securities | Variable Interest Entity, Primary Beneficiary
Assets:
Purchases	0.0
Sales	(0.5)
Issuances	0.0
Settlements	0.0
Purchases, sales, issuances and settlements, net	(0.5)
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	0.0		0.0		0.4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0.1
Transfers into level 3	0.0
Transfers out of level 3	0.0
Amount of total gains (losses) included in our net income relating to assets and liabilities still held as of the reporting date	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets and liabilities still held as of the reporting date	0.0
Asset-backed securities
Assets:
Purchases	0.0	0.0	0.0	0.0
Sales	(0.1)	(0.2)	(0.4)	(0.5)
Issuances	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Purchases, sales, issuances and settlements, net	(0.1)	(0.2)	(0.4)	(0.5)
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	12.6	12.6	12.6	12.6	12.6	12.6	12.4	12.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0	0.0	0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	0.1	0.4	0.4	1.1
Transfers into level 3	0.0	0.0	0.0	0.0
Transfers out of level 3	0.0	0.0	0.0	0.0
Amount of total gains (losses) included in our net income relating to assets and liabilities still held as of the reporting date	0.0	0.0	0.0	0.0
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets and liabilities still held as of the reporting date	0.1		0.4
Non-agency residential mortgage-backed securities
Assets:
Purchases	2.2		2.2
Sales	0.0		0.0
Issuances	0.0		0.0
Settlements	0.0		0.0
Purchases, sales, issuances and settlements, net	2.2		2.2
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	2.2		2.2		0.0	0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in 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
Amount of total gains (losses) included in our net income relating to assets and liabilities 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 and liabilities still held as of the reporting date	0.0		0.0
Total fixed maturities, available for sale
Assets:
Purchases	3.5	0.0	13.2	0.1
Sales	(0.2)	(1.9)	(2.8)	(28.4)
Issuances	0.0	0.0	0.0	0.0
Settlements	0.0	0.0	0.0	0.0
Purchases, sales, issuances and settlements, net	3.3	(1.9)	10.4	(28.3)
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value	138.3	185.1	138.3	185.1	$ 126.8	$ 192.5	165.2	171.6
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings	(1.0)	(1.8)	(1.1)	(4.6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)	2.0	2.7	5.0	11.7
Transfers into level 3	55.8	36.8	82.2	34.7
Transfers out of level 3	(48.6)	(15.9)	(150.7)	0.0
Amount of total gains (losses) included in our net income relating to assets and liabilities still held as of the reporting date	(1.0)	(1.8)	(1.2)	(4.0)
Amount of total gains (losses) included in accumulated other comprehensive income (loss) relating to assets and liabilities still held as of the reporting date	$ 1.3		2.7
Commercial mortgage-backed securities
Assets:
Purchases			4.3
Sales			0.0
Issuances			0.0
Settlements			0.0
Purchases, sales, issuances and settlements, net		0.0	$ 4.3	0.0
Liabilities:
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value		0.0		0.0			$ 15.9	$ 0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings		0.0		0.0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Other Comprehensive Income (Loss)		0.0		0.0
Transfers into level 3		0.0		0.0
Transfers out of level 3		(15.9)		0.0
Amount of total gains (losses) included in our net income relating to assets and liabilities still held as of the reporting date		$ 0.0		$ 0.0