Investments - Amortized Cost and Estimated Fair Value of investments (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	$ 7,475.7	$ 7,295.0
Gross Unrealized Gains	20.8	14.8
Gross Unrealized Losses	(733.1)	(889.5)
Allowance for credit loss, available for sale	20.1	10.7
Fair Value	6,743.3	6,409.6
United States Government and Government Agencies and Authorities
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	702.0	588.6
Gross Unrealized Gains	3.2	0.6
Gross Unrealized Losses	(82.8)	(102.4)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	622.4	486.8
States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	1,437.3	1,457.3
Gross Unrealized Gains	1.8	1.6
Gross Unrealized Losses	(185.7)	(225.4)
Allowance for credit loss, available for sale	0.2	0.3
Fair Value	1,253.2	1,233.2
Foreign Governments
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	10.7	6.5
Gross Unrealized Gains	0.4	0.3
Gross Unrealized Losses	(0.1)	(0.2)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	11.0	6.6
Bonds and Notes
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	4,089.8	4,038.3
Gross Unrealized Gains	11.4	8.9
Gross Unrealized Losses	(433.5)	(518.8)
Allowance for credit loss, available for sale	17.4	8.8
Fair Value	3,650.3	3,519.6
Redeemable Preferred Stocks
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	9.8	9.8
Gross Unrealized Gains	0.4	0.1
Gross Unrealized Losses	0.0	(1.0)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	10.2	8.9
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	850.8	747.8
Gross Unrealized Gains	2.5	2.5
Gross Unrealized Losses	(7.8)	(7.2)
Allowance for credit loss, available for sale	2.5	1.6
Fair Value	843.0	741.5
Other Mortgage- and Asset-backed
Debt Securities, Available-for-sale [Line Items]
Amortized cost of fixed maturities	375.3	446.7
Gross Unrealized Gains	1.1	0.8
Gross Unrealized Losses	(23.2)	(34.5)
Allowance for credit loss, available for sale	0.0	0.0
Fair Value	$ 353.2	$ 413.0

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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