Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,031.9	$ 1,180.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	840.0	944.2
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	97.3	99.3
Operating Segments | Specialty and Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	937.3	1,043.5
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	94.6	137.4
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	675.3	787.9
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	164.7	156.3
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	80.6	82.2
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	5.6	5.9
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 11.1	$ 11.2