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INVESTMENTS - NARRATIVE (Details) $ in Millions	12 Months Ended
INVESTMENTS - NARRATIVE (Details) $ in Millions	Dec. 31, 2024 USD ($) mortgage_loan loan investment state	Dec. 31, 2023 USD ($) investment	Dec. 31, 2022 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale at amortized cost	$ 25,264.3	$ 23,699.2
Estimated fair value	22,840.5	21,506.2
Amortized cost	25,264.3
Amortized cost of fixed maturity investments not accruing investment income	5.7
Carrying value of fixed maturity investments not accruing investment income	4.1
Net realized investment gains (losses)	(49.9)	(69.0)	$ (135.4)
Gain on dissolution of variable interest entities	3.9	0.0	0.0
Change in allowance for credit losses and write-downs	(2.6)	8.1	(52.6)
Value of available for sale securities sold	1,432.0	712.2	1,651.5
Estimated fair value	$ 7,089.3
Percent of fixed maturities	31.00%
Number of additional states greater than specified percentage of mortgage loan balance \| state	0
Assets held by insurance regulators	$ 38.2	$ 38.0
Number of investments in excess of 10% of shareholders' equity \| investment	0	0
Commercial Portfolio Segment
Debt Securities, Available-for-sale [Line Items]
Amortized cost of loans	$ 1,501.4
Fair value of loans	$ 1,344.2
Number of mortgage loans in foreclosure \| mortgage_loan	0
Residential Portfolio Segment \| Financial Asset, Past Due
Debt Securities, Available-for-sale [Line Items]
Amortized cost of loans	$ 15.6
Number of mortgage loans noncurrent \| loan	21
Residential Portfolio Segment \| Financial Asset, Past Due \| Foreclosure
Debt Securities, Available-for-sale [Line Items]
Amortized cost of loans	$ 3.7
Number of mortgage loans noncurrent \| loan	8
Minimum
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	5.00%
California
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	17.00%
Maryland
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	7.00%
Wisconsin
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	6.00%
Utah
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	6.00%
Georgia
Debt Securities, Available-for-sale [Line Items]
Percentage of mortgage loan balance	5.00%
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale	$ 35.7	$ 48.8	70.9
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale		6.7
Other Investments
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale	6.7	3.4	9.1
Residential Mortgage \| Total amortized cost
Debt Securities, Available-for-sale [Line Items]
Mortgage loans	1,018.6
Residential Mortgage \| Estimated fair value
Debt Securities, Available-for-sale [Line Items]
Mortgage loans	1,031.8
Non-agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale			16.5
States and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale			7.5
Fixed indexed annuities - embedded derivative
Debt Securities, Available-for-sale [Line Items]
Increase (decreases) in fair value of certain fixed maturity investments with embedded derivatives	24.4	(8.5)	(45.9)
Reinsurance Contract \| Embedded derivative related to modified coinsurance agreement
Debt Securities, Available-for-sale [Line Items]
Increase (decreases) in fair value of certain fixed maturity investments with embedded derivatives	0.4	0.3	(16.1)
Marketable Securities
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(75.6)	(68.7)	(9.6)
Equity Securities
Debt Securities, Available-for-sale [Line Items]
Net realized investment gains (losses)	(0.4)	(0.2)	(11.2)
Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Estimated fair value	22,840.5	21,506.2
Amortized cost	$ 25,264.3	23,699.2
Continuous unrealized loss position	20.00%
Total fixed maturities, available for sale
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale	$ 54.9	58.9	$ 104.0
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Gross realized losses on sale	12.5
Estimated fair value	$ 2,197.4
Percent of fixed maturities	9.60%
NAIC
Debt Securities, Available-for-sale [Line Items]
Percentage of available-for-sale debt securities	3.60%
Estimated fair value	$ 837.5
Available-for-sale securities, percentage of amortized cost	92.00%
Amortized cost	$ 911.2
Below-investment Grade
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale at amortized cost	$ 1,288.0
Percentage of available-for-sale debt securities	5.10%
Estimated fair value	$ 1,225.5
Available-for-sale securities, percentage of amortized cost	95.00%
Continuous unrealized loss position	20.00%
Below-investment Grade \| Fixed Maturities
Debt Securities, Available-for-sale [Line Items]
Estimated fair value	$ 1,225.5	1,236.7
Amortized cost	1,288.0	1,317.2
Below-investment Grade \| Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Estimated fair value	76.3	71.4
Amortized cost	$ 103.8	$ 100.5