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FAIR VALUE MEASUREMENTS - FAIR VALUE MEASUREMENTS OF FINANCIAL INSTRUMENTS MEASURED ON A RECURRING BASIS - SECURITIES (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021	Jan. 01, 2020
Assets:
Estimated fair value	$ 20,353.4	$ 24,805.4	$ 21,293.1
Equity securities - corporate securities	135.3	131.1
Trading securities	207.9	227.2
Investments held by variable interest entities	1,077.6	1,199.6	$ 1,178.7
Total other invested assets	1,034.7	1,224.0
Assets held in separate accounts	2.7	3.9
Significant unobservable inputs (Level 3)
Assets:
Total assets carried at fair value by category	353.4	170.4
Fair value, Measurements, Recurring
Assets:
Trading securities	207.9	227.2
Investments held by variable interest entities	1,077.6	1,199.6
Derivatives	56.7	227.5
Residential tranches	18.3
Total other invested assets	75.0
Assets held in separate accounts	2.7	3.9
Total assets carried at fair value by category	21,851.9	26,594.7
Liabilities:
Embedded derivatives associated with fixed indexed annuity products (classified as policyholder account liabilities)	1,297.0	1,724.1
Fair value, Measurements, Recurring \| Corporate securities
Assets:
Estimated fair value	11,712.7	15,450.8
Equity securities - corporate securities	135.3	131.1
Fair value, Measurements, Recurring \| United States Treasury securities and obligations of United States government corporations and agencies
Assets:
Estimated fair value	158.7	219.6
Fair value, Measurements, Recurring \| States and political subdivisions
Assets:
Estimated fair value	2,388.5	3,004.2
Fair value, Measurements, Recurring \| Foreign governments
Assets:
Estimated fair value	74.7	98.5
Fair value, Measurements, Recurring \| Asset-backed securities
Assets:
Estimated fair value	1,287.0	1,162.9
Trading securities	15.1	5.8
Fair value, Measurements, Recurring \| Agency residential mortgage-backed securities
Assets:
Estimated fair value	175.0	40.4
Trading securities	0.3	0.4
Fair value, Measurements, Recurring \| Non-agency residential mortgage-backed securities
Assets:
Estimated fair value	1,548.5	2,023.8
Trading securities	60.7	81.0
Fair value, Measurements, Recurring \| Collateralized loan obligations
Assets:
Estimated fair value	785.9	588.3
Fair value, Measurements, Recurring \| Commercial mortgage-backed securities
Assets:
Estimated fair value	2,222.4	2,216.9
Trading securities	131.8	140.0
Fair value, Measurements, Recurring \| Total fixed maturities, available for sale
Assets:
Estimated fair value	20,353.4	24,805.4
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1)
Assets:
Trading securities	0.0	0.0
Investments held by variable interest entities	0.0	0.0
Derivatives	0.0	0.0
Residential tranches	0.0
Total other invested assets	0.0
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	59.6	100.8
Liabilities:
Embedded derivatives associated with fixed indexed annuity products (classified as policyholder account liabilities)	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Corporate securities
Assets:
Estimated fair value	0.0	0.0
Equity securities - corporate securities	59.6	100.8
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| United States Treasury securities and obligations of United States government corporations and agencies
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| States and political subdivisions
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Foreign governments
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Asset-backed securities
Assets:
Estimated fair value	0.0	0.0
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Agency residential mortgage-backed securities
Assets:
Estimated fair value	0.0	0.0
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Non-agency residential mortgage-backed securities
Assets:
Estimated fair value	0.0	0.0
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Collateralized loan obligations
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Commercial mortgage-backed securities
Assets:
Estimated fair value	0.0	0.0
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Quoted prices in active markets for identical assets or liabilities (Level 1) \| Total fixed maturities, available for sale
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2)
Assets:
Trading securities	207.4	210.8
Investments held by variable interest entities	1,077.6	1,197.4
Derivatives	56.7	227.5
Residential tranches	0.0
Total other invested assets	56.7
Assets held in separate accounts	2.7	3.9
Total assets carried at fair value by category	21,438.9	26,323.5
Liabilities:
Embedded derivatives associated with fixed indexed annuity products (classified as policyholder account liabilities)	0.0	0.0
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Corporate securities
Assets:
Estimated fair value	11,584.9	15,361.1
Equity securities - corporate securities	0.0	18.8
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| United States Treasury securities and obligations of United States government corporations and agencies
Assets:
Estimated fair value	158.7	219.6
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| States and political subdivisions
Assets:
Estimated fair value	2,388.5	3,004.2
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Foreign governments
Assets:
Estimated fair value	74.7	98.5
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Asset-backed securities
Assets:
Estimated fair value	1,230.0	1,136.3
Trading securities	15.1	5.8
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Agency residential mortgage-backed securities
Assets:
Estimated fair value	175.0	40.4
Trading securities	0.3	0.4
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Non-agency residential mortgage-backed securities
Assets:
Estimated fair value	1,492.3	2,023.8
Trading securities	60.2	77.5
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Collateralized loan obligations
Assets:
Estimated fair value	782.5	583.3
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Commercial mortgage-backed securities
Assets:
Estimated fair value	2,207.9	2,197.9
Trading securities	131.8	127.1
Fair value, Measurements, Recurring \| Significant other observable inputs (Level 2) \| Total fixed maturities, available for sale
Assets:
Estimated fair value	20,094.5	24,665.1
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3)
Assets:
Trading securities	0.5	16.4
Investments held by variable interest entities	0.0	2.2
Derivatives	0.0	0.0
Residential tranches	18.3
Total other invested assets	18.3
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	353.4	170.4
Liabilities:
Embedded derivatives associated with fixed indexed annuity products (classified as policyholder account liabilities)	1,297.0	1,724.1
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Corporate securities
Assets:
Estimated fair value	127.8	89.7
Equity securities - corporate securities	75.7	11.5
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| United States Treasury securities and obligations of United States government corporations and agencies
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| States and political subdivisions
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Foreign governments
Assets:
Estimated fair value	0.0	0.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Asset-backed securities
Assets:
Estimated fair value	57.0	26.6
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Agency residential mortgage-backed securities
Assets:
Estimated fair value	0.0	0.0
Trading securities	0.0	0.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Non-agency residential mortgage-backed securities
Assets:
Estimated fair value	56.2	0.0
Trading securities	0.5	3.5
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Collateralized loan obligations
Assets:
Estimated fair value	3.4	5.0
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Commercial mortgage-backed securities
Assets:
Estimated fair value	14.5	19.0
Trading securities	0.0	12.9
Fair value, Measurements, Recurring \| Significant unobservable inputs (Level 3) \| Total fixed maturities, available for sale
Assets:
Estimated fair value	$ 258.9	$ 140.3