Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	[1]	$ 4,215.9	$ 4,529.4	$ 5,213.4
Life Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		(7.2)	(19.1)	13.3
Operating Segments
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		3,970.3	4,020.1	4,617.9
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		3,576.4	3,632.5	4,046.4
Operating Segments | Specialty Property & Casualty Insurance | Personal Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		2,851.4	2,977.8	3,496.7
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		725.0	654.7	549.7
Operating Segments | Life Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		393.9	387.6	571.5
Operating Segments | Life Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		328.1	319.2	352.8
Operating Segments | Life Insurance | Accident and Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		22.3	23.1	168.2
Operating Segments | Life Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		43.5	45.3	50.5
Non-Core Operations
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums		$ 245.6	$ 509.3	$ 595.5

[1] 1 Includes a remeasurement loss related to the deferred profit liability within the Life Insurance business of $7.2 million for the year ended December 31, 2024, a remeasurement loss of $19.1 million for the year ended December 31, 2023, and a remeasurement gain of $13.3 million for the year ended December 31, 2022.