Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Remeasurement gain	[1]	$ 1,130.8	$ 1,033.7	$ 2,218.7	$ 2,065.6
Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		(0.9)	(0.9)	(0.7)	(0.7)
Operating Segments
Segment Reporting Information [Line Items]
Remeasurement gain		1,111.3	963.4	2,173.2	1,900.7
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		1,010.8	862.6	1,973.0	1,702.6
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		100.5	100.8	200.2	198.1
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		789.3	691.5	1,543.0	1,366.8
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		221.5	171.1	430.0	335.8
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		84.8	84.4	168.5	165.0
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		5.4	5.6	10.9	11.2
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		10.3	10.8	20.8	21.9
Non-Core Operations
Segment Reporting Information [Line Items]
Remeasurement gain		$ 19.5	$ 70.3	$ 45.5	$ 164.9

[1] 1 Includes a remeasurement loss related to the deferred profit liability within the Life insurance business of $0.9 million for the three months ended June 30, 2025 and 2024, and a remeasurement loss of $0.7 million for the six months ended June 30, 2025 and 2024.