Business Segments - Segment Revenues (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Earned Premiums	[1]	$ 4,215.9	$ 4,529.4	$ 5,213.4
Net Investment Income		407.5	419.7	422.6
Other Income		8.2	7.2	9.2
Total Revenues		4,638.6	4,944.2	5,523.9
Change in Fair Value of Equity and Convertible Securities		(2.7)	4.7	(79.9)
Net Realized Investment Gains (Losses)		13.2	(18.6)	4.3
Impairment Losses		(5.8)	(1.1)	(25.8)
Change in Value of Alternative Energy Partnership Investments		2.3	2.9	(19.9)
Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums		(7.2)	(19.1)	13.3
Operating Segments
Segment Reporting Information [Line Items]
Earned Premiums		3,970.3	4,020.1	4,617.9
Total Revenues		4,337.7	4,388.4	4,965.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums		3,576.4	3,632.5	4,046.4
Net Investment Income		189.6	168.3	140.7
Other Income		4.7	4.5	6.0
Total Revenues		3,772.1	3,806.9	4,183.2
Change in Value of Alternative Energy Partnership Investments		1.4	1.6	(9.9)
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums		393.9	387.6	571.5
Net Investment Income		170.6	193.4	216.5
Other Income		0.5	(0.2)	(0.6)
Total Revenues		565.6	581.5	782.1
Change in Value of Alternative Energy Partnership Investments		0.6	0.7	(5.3)
Non-Core Operations
Segment Reporting Information [Line Items]
Earned Premiums		245.6	509.3	595.5
Total Revenues		282.4	558.4	640.5
Change in Fair Value of Equity and Convertible Securities		(2.7)	4.7	(79.9)
Net Realized Investment Gains (Losses)		13.2	(18.6)	4.3
Impairment Losses		(5.8)	(1.1)	(25.8)
Other		$ 13.8	$ 12.4	$ 19.5

[1] 1 Includes a remeasurement loss related to the deferred profit liability within the Life Insurance business of $7.2 million for the year ended December 31, 2024, a remeasurement loss of $19.1 million for the year ended December 31, 2023, and a remeasurement gain of $13.3 million for the year ended December 31, 2022.