Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	$ 4,529.4	$ 5,213.4	[1]	$ 5,179.2	[1]
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	3,632.5	4,046.4		3,948.5
Operating Segments | Specialty Property & Casualty Insurance | Personal Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	2,977.8	3,496.7		3,533.7
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	654.7	549.7		414.8
Operating Segments | Non-Core Operations
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	509.3	595.5		651.7
Operating Segments | Life Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	387.6	571.5		579.0
Operating Segments | Life Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	319.2	352.8		327.2
Operating Segments | Life Insurance | Accident and Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	23.1	168.2		189.9
Operating Segments | Life Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	45.3	50.5		61.9
Operating Segments | Specialty and Life Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	$ 4,020.1	$ 4,617.9		$ 4,527.5

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.