FAIR VALUE MEASUREMENTS - MEASUREMENTS BY INPUT LEVEL (Details) - USD ($) $ in Millions | Sep. 30, 2015 | Dec. 31, 2014 |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | $ 20,144.5 | $ 20,634.9 |
Trading securities | 257.5 | 244.9 |
Investments held by variable interest entities - corporate securities | 1,488.1 | 1,367.1 |
Assets held in separate accounts | 5 | 5.6 |
Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 3.2 | 3.5 |
Investments held by variable interest entities - corporate securities | 0 | 0 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 237.8 | 221.8 |
Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 224.2 | 212.8 |
Investments held by variable interest entities - corporate securities | 1,488.1 | 1,367.1 |
Assets held in separate accounts | 5 | 5.6 |
Total assets carried at fair value by category | 21,860.9 | 22,039.5 |
Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 30.1 | 28.6 |
Investments held by variable interest entities - corporate securities | 0 | 0 |
Assets held in separate accounts | 0 | 0 |
Total assets carried at fair value by category | 263 | 518.8 |
Corporate securities [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Equity securities - corporate securities | 234 | 216.9 |
Trading securities | 0 | 0 |
Corporate securities [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 13,051.5 | 13,605.1 |
Equity securities - corporate securities | 172.4 | 174.1 |
Trading securities | 21.8 | 24.3 |
Corporate securities [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 146.3 | 365.9 |
Equity securities - corporate securities | 30.9 | 28 |
Trading securities | 0 | 0 |
United States Treasury securities and obligations of United States government corporations and agencies [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
United States Treasury securities and obligations of United States government corporations and agencies [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 162.9 | 168.9 |
Trading securities | 2.7 | 3.7 |
United States Treasury securities and obligations of United States government corporations and agencies [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
States and political subdivisions [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
States and political subdivisions [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,259.4 | 2,242.2 |
States and political subdivisions [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 35.5 |
Debt securities issued by foreign governments [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Debt securities issued by foreign governments [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 16.5 | 1.9 |
Debt securities issued by foreign governments [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Asset-backed securities [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Asset-backed securities [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,459.6 | 1,209.8 |
Trading securities | 21.7 | 24 |
Asset-backed securities [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 40 | 59.2 |
Trading securities | 0 | 0 |
Collateralized debt obligations [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | |
Collateralized debt obligations [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 361.4 | 324.5 |
Trading securities | 2.4 | |
Collateralized debt obligations [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | |
Commercial mortgage-backed securities [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Commercial mortgage-backed securities [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,503 | 1,275.1 |
Trading securities | 149.9 | 131 |
Commercial mortgage-backed securities [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 15.6 | 1.2 |
Trading securities | 30.1 | 28.6 |
Mortgage pass-through securities [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | | 0 |
Mortgage pass-through securities [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 3.4 | 4.2 |
Trading securities | | 0.1 |
Mortgage pass-through securities [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0.1 | 0.4 |
Trading securities | | 0 |
Collateralized mortgage obligations [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Collateralized mortgage obligations [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,124.8 | 1,341 |
Trading securities | 25.7 | 29.7 |
Collateralized mortgage obligations [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Trading securities | 0 | 0 |
Total fixed maturities, available for sale [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 0 | 0 |
Total fixed maturities, available for sale [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 19,942.5 | 20,172.7 |
Total fixed maturities, available for sale [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 202 | 462.2 |
Equity securities [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 3.2 | 3.5 |
Equity securities [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 0 | 0 |
Equity securities [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 0 | 0 |
Derivatives [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 0.6 | 1.4 |
Derivatives [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 28.7 | 107.2 |
Derivatives [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 0 | 0 |
Embedded derivatives associated with fixed index annuity products [Member] | Level 1 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 0 | 0 |
Embedded derivatives associated with fixed index annuity products [Member] | Level 2 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 0 | 0 |
Embedded derivatives associated with fixed index annuity products [Member] | Level 3 [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | 1,079.1 | 1,081.5 |
Estimate of fair value measurement [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 257.5 | 244.9 |
Investments held by variable interest entities - corporate securities | 1,488.1 | 1,367.1 |
Assets held in separate accounts | 5 | 5.6 |
Total assets carried at fair value by category | 22,361.7 | 22,780.1 |
Estimate of fair value measurement [Member] | Corporate securities [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 13,197.8 | 13,971 |
Equity securities - corporate securities | 437.3 | 419 |
Trading securities | 21.8 | 24.3 |
Estimate of fair value measurement [Member] | United States Treasury securities and obligations of United States government corporations and agencies [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 162.9 | 168.9 |
Trading securities | 2.7 | 3.7 |
Estimate of fair value measurement [Member] | States and political subdivisions [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 2,259.4 | 2,277.7 |
Estimate of fair value measurement [Member] | Debt securities issued by foreign governments [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 16.5 | 1.9 |
Estimate of fair value measurement [Member] | Asset-backed securities [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,499.6 | 1,269 |
Trading securities | 21.7 | 24 |
Estimate of fair value measurement [Member] | Collateralized debt obligations [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 361.4 | 324.5 |
Trading securities | 2.4 | |
Estimate of fair value measurement [Member] | Commercial mortgage-backed securities [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,518.6 | 1,276.3 |
Trading securities | 180 | 159.6 |
Estimate of fair value measurement [Member] | Mortgage pass-through securities [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 3.5 | 4.6 |
Trading securities | | 0.1 |
Estimate of fair value measurement [Member] | Collateralized mortgage obligations [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 1,124.8 | 1,341 |
Trading securities | 25.7 | 29.7 |
Estimate of fair value measurement [Member] | Total fixed maturities, available for sale [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Total fixed maturities, available for sale | 20,144.5 | 20,634.9 |
Estimate of fair value measurement [Member] | Equity securities [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Trading securities | 3.2 | 3.5 |
Estimate of fair value measurement [Member] | Derivatives [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Other invested assets - derivatives | 29.3 | 108.6 |
Estimate of fair value measurement [Member] | Embedded derivatives associated with fixed index annuity products [Member] | Fair value, measurements, recurring [Member] | | |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] | | |
Future policy benefits - embedded derivatives associated with fixed index annuity products | $ 1,079.1 | $ 1,081.5 |