Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 1,145.8	$ 1,135.2	$ 1,116.6	$ 1,074.8	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 4,472.4	$ 3,384.4	$ 2,350.0
Operating Segments | Specialty Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									3,078.4	2,027.4	1,005.7
Operating Segments | Specialty Property & Casualty Insurance | Specialty Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									2,825.6	1,889.5	954.3
Operating Segments | Specialty Property & Casualty Insurance | Commercial Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									252.8	137.9	51.4
Operating Segments | Preferred Property & Casualty Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									750.3	730.7	730.3
Operating Segments | Preferred Property & Casualty Insurance | Preferred Automobile
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									470.2	440.2	422.8
Operating Segments | Preferred Property & Casualty Insurance | Homeowners
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									241.3	250.1	264.8
Operating Segments | Preferred Property & Casualty Insurance | Other Personal Lines
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									38.8	40.4	42.7
Operating Segments | Life & Health Insurance
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									643.7	626.3	614.0
Operating Segments | Life & Health Insurance | Life
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									384.6	378.4	379.7
Operating Segments | Life & Health Insurance | Accident & Health
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									190.9	177.5	161.7
Operating Segments | Life & Health Insurance | Property
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									$ 68.2	$ 70.4	$ 72.6