Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 5,253.7	$ 4,672.2	$ 4,472.4
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	3,948.5	3,335.3	3,078.4
Operating Segments | Specialty Property & Casualty Insurance | Specialty Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	3,533.7	3,031.3	2,825.6
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	414.8	304.0	252.8
Operating Segments | Preferred Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	651.7	688.2	750.3
Operating Segments | Preferred Property & Casualty Insurance | Preferred Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	410.5	431.7	470.2
Operating Segments | Preferred Property & Casualty Insurance | Homeowners
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	207.3	220.7	241.3
Operating Segments | Preferred Property & Casualty Insurance | Other Personal Lines
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	33.9	35.8	38.8
Operating Segments | Life & Health Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	653.5	648.7	643.7
Operating Segments | Life & Health Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	401.7	385.7	384.6
Operating Segments | Life & Health Insurance | Accident & Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	189.9	199.3	190.9
Operating Segments | Life & Health Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 61.9	$ 63.7	$ 68.2