FAIR VALUE MEASUREMENTS - MEASUREMENTS BY INPUT LEVEL (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	$ 20,224.8	$ 20,634.9
Trading securities	257.5	244.9
Investments held by variable interest entities - corporate securities	1,565.6	1,367.1
Assets held in separate accounts	5.4	5.6
Level 1 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	3.4	3.5
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	234.6	221.8
Level 2 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	214.2	212.8
Investments held by variable interest entities - corporate securities	1,565.6	1,367.1
Assets held in separate accounts	5.4	5.6
Total assets carried at fair value by category	22,058.6	22,039.5
Level 3 [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Trading securities	39.9	28.6
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	258.2	518.8
Corporate debt 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	0.0
Equity securities - corporate securities	230.2	216.9
Trading securities	0.0	0.0
Corporate debt 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	13,383.2	13,605.1
Equity securities - corporate securities	173.2	174.1
Trading securities	24.9	24.3
Corporate debt 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	133.7	365.9
Equity securities - corporate securities	29.9	28.0
Trading securities	0.0	0.0
US treasury and government [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	0.0
Trading securities	0.0	0.0
US treasury and government [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	166.2	168.9
Trading securities	3.1	3.7
US treasury and government [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	0.0
Trading securities	0.0	0.0
US states and political subdivisions debt 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	0.0
US states and political subdivisions debt 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	2,201.4	2,242.2
US states and political subdivisions debt 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	0.0	35.5
Foreign government debt 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	0.0
Foreign government debt 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	1.9	1.9
Foreign government debt 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	0.0	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	0.0
Trading securities	0.0	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	1,352.7	1,209.8
Trading securities	21.4	24.0
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	53.3	59.2
Trading securities	0.0	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	0.0
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	344.3	324.5
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	0.0
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	0.0
Trading securities	0.0	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	1,381.4	1,275.1
Trading securities	137.8	131.0
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	1.2	1.2
Trading securities	39.9	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	0.0
Trading securities	0.0	0.0
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	3.6	4.2
Trading securities	0.1	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	0.2	0.4
Trading securities	0.0	0.0
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	0.0
Trading securities	0.0	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	1,201.7	1,341.0
Trading securities	26.9	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	0.0
Trading securities	0.0	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	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	20,036.4	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	188.4	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	3.4	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	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	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	1.0	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	63.8	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	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	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	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	1,074.0	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	1,565.6	1,367.1
Assets held in separate accounts	5.4	5.6
Total assets carried at fair value by category	22,551.4	22,780.1
Estimate of fair value measurement [Member] | Corporate debt securities [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	13,516.9	13,971.0
Equity securities - corporate securities	433.3	419.0
Trading securities	24.9	24.3
Estimate of fair value measurement [Member] | US treasury and government [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	166.2	168.9
Trading securities	3.1	3.7
Estimate of fair value measurement [Member] | US states and political subdivisions debt securities [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	2,201.4	2,277.7
Estimate of fair value measurement [Member] | Foreign government debt securities [Member] | Fair value, measurements, recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total fixed maturities, available for sale	1.9	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	1,406.0	1,269.0
Trading securities	21.4	24.0
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	344.3	324.5
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	1,382.6	1,276.3
Trading securities	177.7	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	3.8	4.6
Trading securities	0.1	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	1,201.7	1,341.0
Trading securities	26.9	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	20,224.8	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	3.4	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	64.8	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	$ 1,074.0	$ 1,081.5