Fair Value Disclosures - Carrying Value and Fair Value of the Financial Instruments that are Not Recognized or are Not Carried at Fair Value (Details) - Recurring - USD ($) $ in Millions	Sep. 30, 2026	Dec. 31, 2025
Financial Assets
Other assets	$ 1.0	$ 6.4
Total financial assets	10,879.7	10,549.0
Level 1
Financial Assets
Other assets	0.0	0.0
Total financial assets	1,588.6	1,754.8
Level 2
Financial Assets
Other assets	0.0	0.0
Total financial assets	8,940.7	8,563.0
Level 3
Financial Assets
Other assets	1.0	6.4
Total financial assets	350.4	231.2
Carrying Value
Financial Assets
Commercial mortgage loans on real estate	308.2	324.7
Other investments	16.7	12.4
Other assets	24.3	31.3
Total financial assets	349.2	368.4
Financial Liabilities
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	1.7	1.8
Funds withheld under reinsurance	280.3	266.4
Debt	2,208.7	2,206.9
Total financial liabilities	2,490.7	2,475.1
Fair Value
Financial Assets
Commercial mortgage loans on real estate	302.8	323.1
Other investments	16.7	12.4
Other assets	24.3	31.3
Total financial assets	343.8	366.8
Financial Liabilities
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	1.6	1.8
Funds withheld under reinsurance	280.3	266.4
Debt	2,059.1	2,164.5
Total financial liabilities	2,341.0	2,432.7
Fair Value \| Level 1
Financial Assets
Commercial mortgage loans on real estate	0.0	0.0
Other investments	1.0	1.1
Other assets	0.0	0.0
Total financial assets	1.0	1.1
Financial Liabilities
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	0.0	0.0
Funds withheld under reinsurance	280.3	266.4
Debt	0.0	0.0
Total financial liabilities	280.3	266.4
Fair Value \| Level 2
Financial Assets
Commercial mortgage loans on real estate	0.0	0.0
Other investments	0.0	0.0
Other assets	0.0	0.0
Total financial assets	0.0	0.0
Financial Liabilities
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	0.0	0.0
Funds withheld under reinsurance	0.0	0.0
Debt	2,059.1	2,164.5
Total financial liabilities	2,059.1	2,164.5
Fair Value \| Level 3
Financial Assets
Commercial mortgage loans on real estate	302.8	323.1
Other investments	15.7	11.3
Other assets	24.3	31.3
Total financial assets	342.8	365.7
Financial Liabilities
Policy reserves under investment products (Individual and group annuities, subject to discretionary withdrawal)	1.6	1.8
Funds withheld under reinsurance	0.0	0.0
Debt	0.0	0.0
Total financial liabilities	$ 1.6	$ 1.8

Funds Held under Reinsurance Agreements, Liability, Fair Value Disclosure

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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