Catastrophe Reinsurance - Catastrophe Reinsurance Premiums (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 32.7	$ 42.7	$ 36.1
Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	16.3	32.0	30.3
Specialty Property & Casualty Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	6.1	8.9	7.0
Non-Core Operations | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	9.5	22.5	22.0
Life Insurance | Catastrophe Reinsurance
Liability for Catastrophe Claims [Line Items]
Ceded earned premiums	$ 0.7	$ 0.6	$ 1.3