Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 7,295.0	$ 7,565.8
Gross Unrealized Gains	14.8	54.6
Gross Unrealized Losses	(889.5)	(730.3)
Allowance for credit loss, available for sale	10.7	8.2
Fair Value	6,409.6	6,881.9
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	588.6	594.1
Gross Unrealized Gains	0.6	1.9
Gross Unrealized Losses	(102.4)	(84.5)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	486.8	511.5
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,457.3	1,575.9
Gross Unrealized Gains	1.6	16.3
Gross Unrealized Losses	(225.4)	(189.8)
Allowance for credit loss, available for sale	0.3	0.5
Fair Value	1,233.2	1,401.9
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	6.5	4.4
Gross Unrealized Gains	0.3	0.0
Gross Unrealized Losses	(0.2)	(0.6)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	6.6	3.8
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	4,038.3	4,046.8
Gross Unrealized Gains	8.9	35.5
Gross Unrealized Losses	(518.8)	(383.8)
Allowance for credit loss, available for sale	8.8	7.7
Fair Value	3,519.6	3,690.8
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	9.8	9.0
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	(1.0)	(0.8)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	8.9	8.3
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	747.8	973.6
Gross Unrealized Gains	2.5	0.7
Gross Unrealized Losses	(7.2)	(24.5)
Allowance for credit loss, available for sale	1.6	0.0
Fair Value	741.5	949.8
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	446.7	362.0
Gross Unrealized Gains	0.8	0.1
Gross Unrealized Losses	(34.5)	(46.3)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	$ 413.0	$ 315.8

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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