Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 1,214.0	$ 1,206.5	$ 1,085.3	$ 1,166.4	$ 1,145.8	$ 1,135.2	$ 1,116.6	$ 1,074.8	$ 4,672.2	$ 4,472.4	$ 3,384.4
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									3,335.3	3,078.4	2,027.4
Operating Segments | Specialty Property & Casualty Insurance | Specialty Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									3,031.3	2,825.6	1,889.5
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									304.0	252.8	137.9
Operating Segments | Preferred Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									688.2	750.3	730.7
Operating Segments | Preferred Property & Casualty Insurance | Preferred Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									431.7	470.2	440.2
Operating Segments | Preferred Property & Casualty Insurance | Homeowners
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									220.7	241.3	250.1
Operating Segments | Preferred Property & Casualty Insurance | Other Personal Lines
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									35.8	38.8	40.4
Operating Segments | Life & Health Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									648.7	643.7	626.3
Operating Segments | Life & Health Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									385.7	384.6	378.4
Operating Segments | Life & Health Insurance | Accident & Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									199.3	190.9	177.5
Operating Segments | Life & Health Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									$ 63.7	$ 68.2	$ 70.4