FAIR VALUE MEASUREMENTS - MEASUREMENTS BY INPUT LEVEL (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	$ 21,822.2	$ 19,882.9
Trading securities	307.2	262.1
Investments held by variable interest entities	1,794.4	1,633.6
Assets held in separate accounts	4.7	4.7
Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	307.2	262.1
Investments held by variable interest entities	1,794.4	1,633.6
Assets held in separate accounts	4.7	4.7
Total assets carried at fair value by category	24,344.5	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	1,145.7	1,057.1
Fair value, measurements, recurring [Member] | Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets - derivatives	84.7	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	13,877.1	12,868.5
Equity securities - corporate securities	331.3	463.0
Trading securities	18.7	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	286.7	194.5
Trading securities	0.5	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	2,084.1	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	28.5	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	2,720.8	1,852.2
Trading securities	66.8	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	177.7	186.7
Trading securities	2.3	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	1,579.5	1,605.3
Trading securities	150.3	158.0
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	2.7	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	1,065.1	1,047.4
Trading securities	64.1	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	21,822.2	19,882.9
Fair value, measurements, recurring [Member] | Equity securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	4.5	4.9
Fair value, measurements, recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	4.5	4.9
Investments held by variable interest entities	0.0	0.0
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	131.4	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	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	0.0	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	0.0
Equity securities - corporate securities	126.9	254.9
Trading securities	0.0	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	0.0
Trading securities	0.0	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	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	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	0.0
Trading securities	0.0	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	0.0
Trading securities	0.0	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	0.0
Trading securities	0.0	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	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	0.0
Trading securities	0.0	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	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	4.5	4.9
Fair value, measurements, recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	294.4	217.3
Investments held by variable interest entities	1,794.4	1,633.6
Assets held in separate accounts	4.7	4.7
Total assets carried at fair value by category	23,784.9	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	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	84.7	41.0
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	13,588.6	12,698.1
Equity securities - corporate securities	180.5	176.1
Trading securities	18.7	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	286.7	194.5
Trading securities	0.5	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	2,084.1	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	24.4	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	2,664.3	1,816.3
Trading securities	66.8	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	175.2	186.7
Trading securities	2.3	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	1,547.1	1,604.2
Trading securities	142.0	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	2.7	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	1,053.1	1,047.4
Trading securities	64.1	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	21,426.2	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	0.0
Fair value, measurements, recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	8.3	39.9
Investments held by variable interest entities	0.0	0.0
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	428.2	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	1,145.7	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	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	288.5	170.4
Equity securities - corporate securities	23.9	32.0
Trading securities	0.0	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	0.0
Trading securities	0.0	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	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	0.0
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	56.5	35.9
Trading securities	0.0	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	2.5	0.0
Trading securities	0.0	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	32.4	1.1
Trading securities	8.3	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	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	12.0	0.0
Trading securities	0.0	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	396.0	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	$ 0.0