FAIR VALUE MEASUREMENTS - MEASUREMENTS BY INPUT LEVEL (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017 |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | $ 18,512.4 | $ 22,910.9 |
Trading securities | 239 | 284.6 |
Investments held by variable interest entities | 1,550.5 | 1,526.9 |
Assets held in separate accounts | 5.1 | 5 |
Corporate securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 11,524.5 | 14,958.4 |
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 | 168.7 | 177.7 |
States and political subdivisions | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1,887 | 2,056.3 |
Debt securities issued by foreign governments | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 60.2 | 83.1 |
Asset-backed securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 2,499.1 | 3,254.4 |
Collateralized debt obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 354 | 259.4 |
Commercial mortgage-backed securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1,433.3 | 1,377.5 |
Mortgage pass-through securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1.6 | 2 |
Collateralized mortgage obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 584 | 742.1 |
Equity securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | | 440.6 |
Fair Value, Measurements, Recurring | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 239 | 284.6 |
Investments held by variable interest entities | 1,550.5 | 1,526.9 |
Assets held in separate accounts | 5.1 | 5 |
Total assets carried at fair value by category | 20,786 | 25,338.2 |
Fair Value, Measurements, Recurring | Embedded derivatives associated with fixed index annuity products | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 1,370 | 1,334.8 |
Fair Value, Measurements, Recurring | Derivatives | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 143.5 | 170.2 |
Fair Value, Measurements, Recurring | Corporate securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 11,524.5 | 14,958.4 |
Equity securities - corporate securities | 335.5 | 440.6 |
Trading securities | | 21.6 |
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 | 168.7 | 177.7 |
Trading securities | | 0.5 |
Fair Value, Measurements, Recurring | States and political subdivisions | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1,887 | 2,056.3 |
Fair Value, Measurements, Recurring | Debt securities issued by foreign governments | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 60.2 | 83.1 |
Fair Value, Measurements, Recurring | Asset-backed securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 2,499.1 | 3,254.4 |
Trading securities | 89.7 | 95.8 |
Fair Value, Measurements, Recurring | Collateralized debt obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 354 | 259.4 |
Trading securities | | 2.7 |
Fair Value, Measurements, Recurring | Commercial mortgage-backed securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1,433.3 | 1,377.5 |
Trading securities | 93.2 | 92.5 |
Fair Value, Measurements, Recurring | Mortgage pass-through securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1.6 | 2 |
Fair Value, Measurements, Recurring | Collateralized mortgage obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 584 | 742.1 |
Trading securities | 56.1 | 68.7 |
Fair Value, Measurements, Recurring | Total fixed maturities, available for sale | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 18,512.4 | 22,910.9 |
Fair Value, Measurements, Recurring | Equity securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | | 2.8 |
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 | 2.8 |
Investments held by variable interest entities | 0 | 0 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 216.3 | 290.6 |
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] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 0 | 0 |
Fair Value, Measurements, Recurring | Quoted prices in active markets for identical assets or liabilities (Level 1) | 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 | 216.3 | 287.8 |
Trading securities | | 0 |
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 |
Trading securities | | 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) | Debt securities issued by 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) | Collateralized debt obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 0 | 0 |
Trading securities | | 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) | Mortgage pass-through securities | | |
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) | Collateralized mortgage obligations | | |
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 | Quoted prices in active markets for identical assets or liabilities (Level 1) | Equity securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | | 2.8 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 239 | 281.8 |
Investments held by variable interest entities | 1,550.5 | 1,522 |
Assets held in separate accounts | 5.1 | 5 |
Total assets carried at fair value by category | 20,394.3 | 24,763 |
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] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 0 | 0 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Derivatives | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 143.5 | 170.2 |
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 | 11,371.7 | 14,728 |
Equity securities - corporate securities | 109.7 | 131.6 |
Trading securities | | 21.6 |
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 | 168.7 | 177.7 |
Trading securities | | 0.5 |
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 | 1,887 | 2,056.3 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Debt securities issued by foreign governments | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 59.2 | 79.2 |
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 | 2,487 | 3,230.2 |
Trading securities | 89.7 | 95.8 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Collateralized debt obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 354 | 259.4 |
Trading securities | | 2.7 |
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 | 1,433.3 | 1,377.5 |
Trading securities | 93.2 | 92.5 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Mortgage pass-through securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1.6 | 2 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Collateralized mortgage obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 584 | 742.1 |
Trading securities | 56.1 | 68.7 |
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 | 18,346.5 | 22,652.4 |
Fair Value, Measurements, Recurring | Significant other observable inputs (Level 2) | Equity securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | | 0 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 0 | 0 |
Investments held by variable interest entities | 0 | 4.9 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 175.4 | 284.6 |
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] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 1,370 | 1,334.8 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | 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 | 152.8 | 230.4 |
Equity securities - corporate securities | 9.5 | 21.2 |
Trading securities | | 0 |
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 |
Trading securities | | 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) | Debt securities issued by foreign governments | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 1 | 3.9 |
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 | 12.1 | 24.2 |
Trading securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Collateralized debt obligations | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 0 | 0 |
Trading securities | | 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 | 0 | 0 |
Trading securities | 0 | 0 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Mortgage pass-through securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | 0 | 0 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Collateralized mortgage obligations | | |
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) | Total fixed maturities, available for sale | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Estimated fair value | $ 165.9 | 258.5 |
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3) | Equity securities | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | | $ 0 |