Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,116.6	$ 658.1	$ 2,191.4	$ 1,267.9
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	188.5	181.1	374.1	357.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	766.0	320.0	1,495.3	598.4
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	162.1	157.0	322.0	311.6
Operating Segments | Personal Automobile | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	117.9	108.7	233.3	213.6
Operating Segments | Homeowners | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	60.8	62.2	121.1	124.0
Operating Segments | Other Personal Lines | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	9.8	10.2	19.7	20.3
Operating Segments | Specialty Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	703.7	307.5	1,373.3	573.7
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	62.3	12.5	122.0	24.7
Operating Segments | Life | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	97.0	95.4	192.8	189.1
Operating Segments | Accident and Health | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	47.9	43.8	94.8	87.1
Operating Segments | Property | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	$ 17.2	$ 17.8	$ 34.4	$ 35.4