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Business Segments - Earned Premiums by Product Line (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Remeasurement gain	[1]	$ 1,133.3	$ 1,068.5	$ 3,352.0	$ 3,134.1
Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		(0.9)	(0.5)	(1.6)	(1.2)
Operating Segments
Segment Reporting Information [Line Items]
Remeasurement gain		1,117.1	1,019.6	3,290.3	2,920.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		1,017.3	919.0	2,990.3	2,621.6
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		99.8	100.6	300.0	298.7
Operating Segments | Personal Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		785.1	731.3	2,328.1	2,098.1
Operating Segments | Commercial Automobile | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		232.2	187.7	662.2	523.5
Operating Segments | Life | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		84.1	84.2	252.6	249.2
Operating Segments | Accident and Health | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		5.5	5.6	16.4	16.8
Operating Segments | Property | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		10.2	10.8	31.0	32.7
Non-Core Operations
Segment Reporting Information [Line Items]
Remeasurement gain		$ 16.2	$ 48.9	$ 61.7	$ 213.8
[1] 1 Includes a remeasurement loss related to the deferred profit liability within the Life insurance business of $0.9 million and $0.5 million for the three months ended September 30, 2025 and 2024, respectively, and a remeasurement loss of $1.6 million and $1.2 million for the nine months ended September 30, 2025 and 2024, respectively.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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