FAIR VALUE MEASUREMENTS - FAIR VALUE MEASUREMENTS OF FINANCIAL INSTRUMENTS MEASURED ON A RECURRING BASIS - SECURITIES (Details) - USD ($) $ in Millions | Dec. 31, 2021 | Dec. 31, 2020 | Jan. 01, 2020 | Jan. 01, 2019 |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | $ 24,805.4 | | $ 21,293.1 | $ 18,443.7 |
Equity securities - corporate securities | 131.1 | $ 151.2 | | |
Trading securities | 227.2 | 232 | | |
Investments held by variable interest entities | 1,199.6 | 1,189.4 | $ 1,178.7 | |
Assets held in separate accounts | 3.9 | 4.2 | | |
Significant unobservable inputs (Level 3) | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Total assets carried at fair value by category | 170.4 | 212.5 | | |
Fair value, Measurements, Recurring | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Trading securities | 227.2 | 232 | | |
Investments held by variable interest entities | 1,199.6 | 1,189.4 | | |
Assets held in separate accounts | 3.9 | 4.2 | | |
Total assets carried at fair value by category | 26,594.7 | 25,177.1 | | |
Fair value, Measurements, Recurring | Embedded Derivatives Associated with Fixed Index Annuity Products | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Embedded derivatives associated with fixed index annuity products (classified as policyholder account liabilities) | 1,724.1 | 1,644.5 | | |
Fair value, Measurements, Recurring | Other Invested Assets - Derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Other invested assets - derivatives | 227.5 | 216.7 | | |
Fair value, Measurements, Recurring | Corporate securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 15,450.8 | 14,739.2 | | |
Equity securities - corporate securities | 131.1 | 151.2 | | |
Fair value, Measurements, Recurring | United States Treasury securities and obligations of United States government corporations and agencies | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 219.6 | 235.5 | | |
Fair value, Measurements, Recurring | States and political subdivisions | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 3,004.2 | 2,653.9 | | |
Fair value, Measurements, Recurring | Foreign governments | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 98.5 | 102.8 | | |
Fair value, Measurements, Recurring | Asset-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 1,162.9 | 1,062.1 | | |
Trading securities | 5.8 | 10.4 | | |
Fair value, Measurements, Recurring | Agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 40.4 | 58.4 | | |
Trading securities | 0.4 | 0.4 | | |
Fair value, Measurements, Recurring | Non-agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 2,023.8 | 2,092.6 | | |
Trading securities | 81 | 97.9 | | |
Fair value, Measurements, Recurring | Collateralized loan obligations | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 588.3 | 458.9 | | |
Fair value, Measurements, Recurring | Commercial mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 2,216.9 | 1,980.2 | | |
Trading securities | 140 | 123.3 | | |
Fair value, Measurements, Recurring | Total fixed maturities, available for sale | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 24,805.4 | 23,383.6 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Trading securities | 0 | 0 | | |
Investments held by variable interest entities | 0 | 0 | | |
Assets held in separate accounts | 0 | 0 | | |
Total assets carried at fair value by category | 100.8 | 104.6 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Embedded Derivatives Associated with Fixed Index Annuity Products | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Embedded derivatives associated with fixed index annuity products (classified as policyholder account liabilities) | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Other Invested Assets - Derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Other invested assets - derivatives | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Corporate securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Equity securities - corporate securities | 100.8 | 104.6 | | |
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 | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | States and political subdivisions | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Foreign governments | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Asset-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Non-agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Collateralized loan obligations | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Commercial mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | Total fixed maturities, available for sale | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Trading securities | 210.8 | 209.1 | | |
Investments held by variable interest entities | 1,197.4 | 1,189.4 | | |
Assets held in separate accounts | 3.9 | 4.2 | | |
Total assets carried at fair value by category | 26,323.5 | 24,860 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Embedded Derivatives Associated with Fixed Index Annuity Products | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Embedded derivatives associated with fixed index annuity products (classified as policyholder account liabilities) | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Other Invested Assets - Derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Other invested assets - derivatives | 227.5 | 216.7 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Corporate securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 15,361.1 | 14,592.3 | | |
Equity securities - corporate securities | 18.8 | 19.8 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | United States Treasury securities and obligations of United States government corporations and agencies | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 219.6 | 235.5 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | States and political subdivisions | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 3,004.2 | 2,653.9 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Foreign governments | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 98.5 | 102.8 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Asset-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 1,136.3 | 1,047.8 | | |
Trading securities | 5.8 | 10.4 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 40.4 | 58.4 | | |
Trading securities | 0.4 | 0.4 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Non-agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 2,023.8 | 2,091 | | |
Trading securities | 77.5 | 92 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Collateralized loan obligations | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 583.3 | 458.9 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Commercial mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 2,197.9 | 1,980.2 | | |
Trading securities | 127.1 | 106.3 | | |
Fair value, Measurements, Recurring | Significant other observable inputs (Level 2) | Total fixed maturities, available for sale | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 24,665.1 | 23,220.8 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Trading securities | 16.4 | 22.9 | | |
Investments held by variable interest entities | 2.2 | 0 | | |
Assets held in separate accounts | 0 | 0 | | |
Total assets carried at fair value by category | 170.4 | 212.5 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Embedded Derivatives Associated with Fixed Index Annuity Products | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Embedded derivatives associated with fixed index annuity products (classified as policyholder account liabilities) | 1,724.1 | 1,644.5 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Other Invested Assets - Derivatives | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Other invested assets - derivatives | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Corporate securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 89.7 | 146.9 | | |
Equity securities - corporate securities | 11.5 | 26.8 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | United States Treasury securities and obligations of United States government corporations and agencies | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | States and political subdivisions | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Foreign governments | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Asset-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 26.6 | 14.3 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 0 | | |
Trading securities | 0 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Non-agency residential mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 0 | 1.6 | | |
Trading securities | 3.5 | 5.9 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Collateralized loan obligations | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 5 | 0 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Commercial mortgage-backed securities | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | 19 | 0 | | |
Trading securities | 12.9 | 17 | | |
Fair value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Total fixed maturities, available for sale | | | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | | | |
Estimated fair value | $ 140.3 | $ 162.8 | | |