Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 5,266.3	$ 5,253.7	$ 4,672.2
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	4,046.4	3,948.5	3,335.3
Operating Segments | Specialty Property & Casualty Insurance | Personal Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	3,496.7	3,533.7	3,031.3
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	549.7	414.8	304.0
Operating Segments | Preferred Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	595.5	651.7	688.2
Operating Segments | Preferred Property & Casualty Insurance | Preferred Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	363.7	410.5	431.7
Operating Segments | Preferred Property & Casualty Insurance | Homeowners
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	200.0	207.3	220.7
Operating Segments | Preferred Property & Casualty Insurance | Other Personal Lines
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	31.8	33.9	35.8
Operating Segments | Life & Health Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	624.4	653.5	648.7
Operating Segments | Life & Health Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	405.7	401.7	385.7
Operating Segments | Life & Health Insurance | Accident & Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	168.2	189.9	199.3
Operating Segments | Life & Health Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 50.5	$ 61.9	$ 63.7