FAIR VALUE MEASUREMENTS (FAIR VALUE ASSETS AND LIABILITIES MEASURED ON A RECURRING BASIS, UNOBSERVABLE INPUT RECONCILIATION) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liabilities:
Cash and cash equivalents - unrestricted	$ 656.6	$ 757.3	$ 668.2	$ 796.7
Cash and cash equivalents held by variable interest entities	62.4	178.9
Future policy benefits - embedded derivatives associated with fixed index annuity products
Liabilities:
Purchases, sales, issuances and settlements, net	(62.0)	(267.5)
Corporate securities
Assets:
Purchases, sales, issuances and settlements, net	(24.6)	(70.4)
Debt securities issued by foreign governments
Assets:
Purchases, sales, issuances and settlements, net	(2.9)
Asset-backed Securities
Assets:
Purchases, sales, issuances and settlements, net	(11.5)	(4.3)
Collateralized debt obligations
Assets:
Purchases, sales, issuances and settlements, net		(2.5)
Commercial mortgage-backed securities
Assets:
Purchases, sales, issuances and settlements, net		(1.2)
Total fixed maturities, available for sale
Assets:
Purchases, sales, issuances and settlements, net	(39.0)	(78.4)
Equity securities - corporate securities
Assets:
Purchases, sales, issuances and settlements, net	(10.9)	(8.5)
Investments Held By Variable Interest Entities | Corporate Securities Held By Variable Interest Entities
Assets:
Purchases, sales, issuances and settlements, net		4.9
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring
Liabilities:
Loans receivable, fair value disclosure	119.7	116.0
Company-owned life insurance	0.0	0.0
Cash and cash equivalents - unrestricted	0.0	0.0
Cash and cash equivalents held by variable interest entities	0.0	0.0
Liabilities for interest sensitive products, excluding embedded derivatives	11,594.1	11,220.7
Investment borrowings, fair value disclosure	0.0	0.0
Borrowings related to variable interest entities, fair value disclosure	0.0	0.0
Notes payable, fair value disclosure	0.0	0.0
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Future policy benefits - embedded derivatives associated with fixed index annuity products
Liabilities:
Beginning balance	(1,334.8)	(1,092.3)
Purchases, sales, issuances and settlements, net	(62.0)	(267.5)
Total realized and unrealized gains (losses) included in net income	107.8	25.0
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	(1,289.0)	(1,334.8)
Amount of total gains (losses) for the year included in our net income related to liabilities still held as of the reporting date	107.8	25.0
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Corporate securities
Assets:
Beginning balance	230.4	258.5
Purchases, sales, issuances and settlements, net	(24.6)	(70.4)
Total realized and unrealized gains (losses) included in net income	0.2	5.8
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(5.3)	5.3
Transfers into level 3	12.7	31.2
Transfers out of level 3	(54.8)	0.0
Ending balance	158.6	230.4
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(0.5)	(8.0)
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Debt securities issued by foreign governments
Assets:
Beginning balance	3.9	3.9
Purchases, sales, issuances and settlements, net	(2.9)	0.0
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)	0.1	0.0
Transfers into level 3	0.0	0.0
Transfers out of level 3	0.0	0.0
Ending balance	1.0	3.9
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	0.0
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Asset-backed Securities
Assets:
Beginning balance	24.2	60.4
Purchases, sales, issuances and settlements, net	(11.5)	(4.3)
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)	(0.7)	0.7
Transfers into level 3	0.0	0.0
Transfers out of level 3	0.0	(32.6)
Ending balance	12.0	24.2
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
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Collateralized debt obligations
Assets:
Beginning balance	0.0	5.4
Purchases, sales, issuances and settlements, net		(2.5)
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)		0.0
Transfers into level 3		0.0
Transfers out of level 3		(2.9)
Ending balance		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
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Commercial mortgage-backed securities
Assets:
Beginning balance	0.0	32.0
Purchases, sales, issuances and settlements, net		(1.2)
Total realized and unrealized gains (losses) included in net income		0.1
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)		(0.1)
Transfers into level 3		0.0
Transfers out of level 3		(30.8)
Ending balance		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
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Total fixed maturities, available for sale
Assets:
Beginning balance	258.5	360.2
Purchases, sales, issuances and settlements, net	(39.0)	(78.4)
Total realized and unrealized gains (losses) included in net income	0.1	5.9
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	(5.9)	5.9
Transfers into level 3	12.7	31.2
Transfers out of level 3	(54.8)	(66.3)
Ending balance	171.6	258.5
Amount of total gains (losses) for the year included in our net income relating to assets still held as of the reporting date	(0.5)	(8.0)
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Equity securities - corporate securities
Assets:
Beginning balance	21.2	25.2
Purchases, sales, issuances and settlements, net	(10.9)	(8.5)
Total realized and unrealized gains (losses) included in net income	(0.8)	6.3
Total realized and unrealized gains (losses) included in accumulated other comprehensive income (loss)	0.0	(1.8)
Transfers into level 3	0.0	0.0
Transfers out of level 3	0.0	0.0
Ending balance	9.5	21.2
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
Fair Value, Inputs, Level 3 | Fair value, measurements, recurring | Investments Held By Variable Interest Entities | Corporate Securities Held By Variable Interest Entities
Assets:
Beginning balance	4.9	0.0
Purchases, sales, issuances and settlements, net	0.0	4.9
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)	0.0	0.0
Transfers into level 3	0.0	0.0
Transfers out of level 3	(4.9)	0.0
Ending balance	0.0	4.9
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	$ 0.0