FAIR VALUE MEASUREMENTS - MEASUREMENTS BY INPUT LEVEL (Details) - USD ($) $ in Millions | Jun. 30, 2016 | Dec. 31, 2015 |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | $ 20,989.8 | $ 19,882.9 |
Trading securities | 289.3 | 262.1 |
Investments held by variable interest entities | 1,807.8 | 1,633.6 |
Assets held in separate accounts | 4.6 | 4.7 |
Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 289.3 | 262.1 |
Investments held by variable interest entities | 1,807.8 | 1,633.6 |
Assets held in separate accounts | 4.6 | 4.7 |
Total assets carried at fair value by category | 23,699.5 | 22,288.9 |
Fair value, measurements, recurring [Member] | Embedded derivatives associated with fixed index annuity products [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 1,127 | 1,057.1 |
Fair value, measurements, recurring [Member] | Derivatives [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 66.8 | 42.6 |
Fair value, measurements, recurring [Member] | Corporate securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 13,570.6 | 12,868.5 |
Equity securities - corporate securities | 541.2 | 463 |
Trading securities | 20 | 21.5 |
Fair value, measurements, recurring [Member] | United States Treasury securities and obligations of United States government corporations and agencies [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 212.1 | 194.5 |
Trading securities | 1.6 | 1.9 |
Fair value, measurements, recurring [Member] | States and political subdivisions [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,111.8 | 2,104.2 |
Fair value, measurements, recurring [Member] | Debt securities issued by foreign governments [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 42.4 | 20.7 |
Fair value, measurements, recurring [Member] | Asset-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,423.6 | 1,852.2 |
Trading securities | 50.2 | 35.5 |
Fair value, measurements, recurring [Member] | Collateralized debt obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 146.1 | 186.7 |
Trading securities | 2 | 2.1 |
Fair value, measurements, recurring [Member] | Commercial mortgage-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,476 | 1,605.3 |
Trading securities | 150.9 | 158 |
Fair value, measurements, recurring [Member] | Mortgage pass-through securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2.8 | 3.4 |
Fair value, measurements, recurring [Member] | Collateralized mortgage obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,004.4 | 1,047.4 |
Trading securities | 59.5 | 38.2 |
Fair value, measurements, recurring [Member] | Total fixed maturities, available for sale [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 20,989.8 | 19,882.9 |
Fair value, measurements, recurring [Member] | Equity securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 5.1 | 4.9 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 5.1 | 4.9 |
Investments held by variable interest entities | 0 | 0 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 351 | 261.4 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Embedded derivatives associated with fixed index annuity products [Member] | | |
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 [Member] | Level 1 [Member] | Derivatives [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 0 | 1.6 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Corporate securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Equity securities - corporate securities | 345.9 | 254.9 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | United States Treasury securities and obligations of United States government corporations and agencies [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | States and political subdivisions [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Debt securities issued by foreign governments [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Asset-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Collateralized debt obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Commercial mortgage-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Mortgage pass-through securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Collateralized mortgage obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Total fixed maturities, available for sale [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 1 [Member] | Equity securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 5.1 | 4.9 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 284.2 | 217.3 |
Investments held by variable interest entities | 1,807.8 | 1,633.6 |
Assets held in separate accounts | 4.6 | 4.7 |
Total assets carried at fair value by category | 23,108.2 | 21,748.1 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Embedded derivatives associated with fixed index annuity products [Member] | | |
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 [Member] | Level 2 [Member] | Derivatives [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 66.8 | 41 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Corporate securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 13,396 | 12,698.1 |
Equity securities - corporate securities | 173.9 | 176.1 |
Trading securities | 20 | 21.5 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | United States Treasury securities and obligations of United States government corporations and agencies [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 212.1 | 194.5 |
Trading securities | 1.6 | 1.9 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | States and political subdivisions [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,111.8 | 2,104.2 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Debt securities issued by foreign governments [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 38.3 | 20.7 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Asset-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,384.5 | 1,816.3 |
Trading securities | 50.2 | 35.5 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Collateralized debt obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 146.1 | 186.7 |
Trading securities | 2 | 2.1 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Commercial mortgage-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,474.9 | 1,604.2 |
Trading securities | 150.9 | 118.1 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Mortgage pass-through securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2.8 | 3.3 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Collateralized mortgage obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,004.4 | 1,047.4 |
Trading securities | 59.5 | 38.2 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Total fixed maturities, available for sale [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 20,770.9 | 19,675.4 |
Fair value, measurements, recurring [Member] | Level 2 [Member] | Equity securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 0 | 39.9 |
Investments held by variable interest entities | 0 | 0 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 240.3 | 279.4 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Embedded derivatives associated with fixed index annuity products [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 1,127 | 1,057.1 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Derivatives [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Corporate securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 174.6 | 170.4 |
Equity securities - corporate securities | 21.4 | 32 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | United States Treasury securities and obligations of United States government corporations and agencies [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | States and political subdivisions [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Debt securities issued by foreign governments [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 4.1 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Asset-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 39.1 | 35.9 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Collateralized debt obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Commercial mortgage-backed securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1.1 | 1.1 |
Trading securities | 0 | 39.9 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Mortgage pass-through securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0.1 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Collateralized mortgage obligations [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Total fixed maturities, available for sale [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 218.9 | 207.5 |
Fair value, measurements, recurring [Member] | Level 3 [Member] | Equity securities [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | $ 0 | $ 0 |