FAIR VALUE MEASUREMENTS (FAIR VALUE MEASUREMENTS OF FINANCIAL INSTRUMENTS MEASURED ON A RECURRING BASIS - SECURITIES) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	$ 21,096.2	$ 19,882.9
Trading securities	363.4	262.1
Investments held by variable interest entities - corporate securities	1,724.3	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	363.4	262.1
Investments held by variable interest entities - corporate securities	1,724.3	1,633.6
Assets held in separate accounts	4.7	4.7
Total assets carried at fair value by category	23,884.7	22,288.9
Fair value, measurements, recurring [Member] | Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets - derivatives	111.9	42.6
Fair value, measurements, recurring [Member] | Embedded derivatives associated with fixed index annuity products [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities for insurance products	1,092.3	1,057.1
Fair value, measurements, recurring [Member] | Corporate securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	13,510.9	12,868.5
Equity securities - corporate securities	584.2	463.0
Trading securities	19.0	21.5
Fair value, measurements, recurring [Member] | US treasury and government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	164.3	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	1,988.9	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	36.9	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	2,710.3	1,852.2
Trading securities	94.3	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	230.7	186.7
Trading securities	2.4	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	1,536.2	1,605.3
Trading securities	163.9	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	2.5	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	915.5	1,047.4
Trading securities	78.4	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	21,096.2	19,882.9
Fair value, measurements, recurring [Member] | Equity securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	4.9	4.9
Fair value, measurements, recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	4.9	4.9
Investments held by variable interest entities - corporate securities	0.0	0.0
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	364.8	261.4
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] | Embedded derivatives associated with fixed index annuity products [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities for insurance products	0.0	0.0
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	359.9	254.9
Trading securities	0.0	0.0
Fair value, measurements, recurring [Member] | Level 1 [Member] | US treasury and government [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	4.9	4.9
Fair value, measurements, recurring [Member] | Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	358.5	217.3
Investments held by variable interest entities - corporate securities	1,724.3	1,633.6
Assets held in separate accounts	4.7	4.7
Total assets carried at fair value by category	23,134.5	21,748.1
Fair value, measurements, recurring [Member] | Level 2 [Member] | Derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Other invested assets - derivatives	111.9	41.0
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]
Total liabilities for insurance products	0.0	0.0
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	13,252.4	12,698.1
Equity securities - corporate securities	199.1	176.1
Trading securities	19.0	21.5
Fair value, measurements, recurring [Member] | Level 2 [Member] | US treasury and government [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	164.3	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	1,988.9	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	33.0	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	2,649.9	1,816.3
Trading securities	94.3	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	225.3	186.7
Trading securities	2.4	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	1,504.2	1,604.2
Trading securities	163.9	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	2.5	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	915.5	1,047.4
Trading securities	78.4	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	20,736.0	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	0.0	39.9
Investments held by variable interest entities - corporate securities	0.0	0.0
Assets held in separate accounts	0.0	0.0
Total assets carried at fair value by category	385.4	279.4
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] | Embedded derivatives associated with fixed index annuity products [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total liabilities for insurance products	1,092.3	1,057.1
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	258.5	170.4
Equity securities - corporate securities	25.2	32.0
Trading securities	0.0	0.0
Fair value, measurements, recurring [Member] | Level 3 [Member] | US treasury and government [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	3.9	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	60.4	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	5.4	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	32.0	1.1
Trading securities	0.0	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	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	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	360.2	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