Business Segments - Segment Revenues (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
Net Investment Income		95.9	93.0	197.1	193.4
Other Income		3.1	3.0	5.7	5.2
Total Revenues		1,225.6	1,129.9	2,418.6	2,272.9
Change in Fair Value of Equity and Convertible Securities		(0.5)	(1.2)	(0.4)	2.2
Impairment Losses		3.6	0.1	3.3	1.6
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
Total Revenues		1,208.6	1,042.3	2,371.4	2,066.7
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		1,010.8	862.6	1,973.0	1,702.6
Net Investment Income		49.6	46.6	100.1	87.7
Other Income		2.7	1.6	4.0	3.0
Total Revenues		1,063.1	910.8	2,077.1	1,793.3
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Remeasurement gain		100.5	100.8	200.2	198.1
Net Investment Income		44.7	30.5	93.1	74.8
Other Income		0.3	0.2	1.0	0.5
Total Revenues		145.5	131.5	294.3	273.4
Non-Core Operations
Segment Reporting Information [Line Items]
Remeasurement gain		19.5	70.3	45.5	164.9
Total Revenues		21.3	83.8	49.2	190.0
Change in Fair Value of Equity and Convertible Securities		(0.5)	(1.2)	(0.4)	2.2
Net Realized Investment (Losses) Gains		(0.1)	1.5	0.8	8.1
Impairment Losses		(3.6)	(0.1)	3.3	1.6
Other		$ (0.1)	$ 3.6	$ 0.9	$ 7.5

[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.