Business Segments - Segment Revenues (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
Net Investment Income		101.2	100.4
Other Income		2.6	2.2
Total Revenues		1,193.0	1,143.0
Change in Fair Value of Equity and Convertible Securities		0.1	3.4
Impairment Losses		(0.3)	1.5
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
Total Revenues		1,162.8	1,024.4
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		962.2	840.0
Net Investment Income		50.5	41.1
Other Income		1.3	1.4
Total Revenues		1,014.0	882.5
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		99.7	97.3
Net Investment Income		48.4	44.3
Other Income		0.7	0.3
Total Revenues		148.8	141.9
Non-Core Operations
Segment Reporting Information [Line Items]
Remeasurement gain		26.0	94.6
Total Revenues		27.9	106.2
Change in Fair Value of Equity and Convertible Securities		0.1	3.4
Net Realized Investment Gains		0.9	6.6
Impairment Losses		0.3	(1.5)
Other		$ 1.0	$ 3.9

[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.