Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Remeasurement gain	[1]	$ 1,087.9	$ 1,031.9
Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		0.2
Operating Segments
Segment Reporting Information [Line Items]
Remeasurement gain		1,061.9	937.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		962.2	840.0
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		99.7	97.3
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		753.7	675.3
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		208.5	164.7
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		83.7	80.6
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		5.5	5.6
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		10.5	11.1
Non-Core Operations
Segment Reporting Information [Line Items]
Remeasurement gain		$ 26.0	$ 94.6

[1] 1 Includes a remeasurement gain related to the deferred profit liability within the Life insurance business of $0.2 million for the three months ended March 31, 2025 and 2024.