Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 605.9	$ 598.2	$ 582.5	$ 563.4	$ 3,384.4	$ 2,350.0	$ 2,220.0
Operating Segments [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									3,384.4	2,350.0	2,220.0
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									730.7	730.3	741.5
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									250.1		271.9
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Other Personal Lines [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									40.4	42.7	45.0
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									2,027.4	1,005.7	873.3
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member] | Specialty Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									1,889.5	954.3	820.0
Operating Segments [Member] | Specialty Property & Casualty Insurance [Member] | Commercial Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									137.9	51.4	53.3
Operating Segments [Member] | Life and Health Insurance [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									626.3	614.0	605.2
Operating Segments [Member] | Life and Health Insurance [Member] | Life [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									378.4	379.7	381.6
Operating Segments [Member] | Life and Health Insurance [Member] | Accidental and Health [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									177.5	161.7	149.4
Operating Segments [Member] | Life and Health Insurance [Member] | Property [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									70.4	72.6	74.2
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Preferred Automobile [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums									$ 440.2	422.8	$ 424.6
Operating Segments [Member] | Preferred Property & Casualty Insurance [Member] | Homeowners [Member]
Entity-Wide Information, Revenue from External Customer [Line Items]
Earned Premiums										$ 264.8