Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	$ 1,074.8	$ 609.8
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Preferred Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	115.4	104.9
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums		61.8
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	185.6	176.8
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	60.3
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Other Personal Property and Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	9.9	10.1
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	729.3
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member] | Specialty Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	669.6	266.2
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member] | Commercial Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	59.7	12.2
Operating Segments [Member] | Life and Health Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	159.9	154.6
Operating Segments [Member] | Life and Health Insurance [Member] | Life [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	95.8	93.7
Operating Segments [Member] | Life and Health Insurance [Member] | Accidental and Health [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	46.9	43.3
Operating Segments [Member] | Life and Health Insurance [Member] | Property [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Total Earned Premiums	$ 17.2	$ 17.6