Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022		Dec. 31, 2021
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	$ 4,529.4	$ 5,213.4	[1]	$ 5,179.2	[1]
Net Investment Income	419.7	422.6	[1]	427.3	[1]
Other Income	7.2	9.2	[1]	4.8	[1]
Total Revenues	4,944.2	5,523.9	[1]	5,718.5	[1]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	4.7	(79.9)	[1]	114.6	[1]
Net Realized Investment (Losses) Gains	(18.6)	4.3	[1]	64.8	[1]
Impairment Losses	(1.1)	(25.8)	[1]	(11.0)	[1]
Change in Value of Alternative Energy Partnership Investments	2.9	(19.9)	[1]	(61.2)	[1]
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	4,388.4	4,965.3		4,840.7
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	3,632.5	4,046.4		3,948.5
Net Investment Income	168.3	140.7		152.5
Other Income	4.5	6.0		4.1
Total Revenues	3,806.9	4,183.2		4,076.1
Change in Value of Alternative Energy Partnership Investments	1.6	(9.9)		(29.0)
Operating Segments | Non-Core Operations
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	509.3	595.5		651.7
Total Revenues	558.4	640.5		704.0
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $84.2, 2022 - $60.2 and 2021 -$80.7)	387.6	571.5		579.0
Net Investment Income	193.4	216.5		202.7
Other Income	(0.2)	(0.6)		(1.3)
Total Revenues	581.5	782.1		764.6
Change in Value of Alternative Energy Partnership Investments	0.7	(5.3)		(15.8)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	4.7	(79.9)		114.6
Net Realized Investment (Losses) Gains	(18.6)	4.3		64.8
Impairment Losses	(1.1)	(25.8)		(11.0)
Other	$ 12.4	$ 19.5		$ 5.4

[1] As of January 1, 2023, the Company adopted ASU 2018-12 using the modified retrospective method applied as of the transition date of January 1, 2021. Prior period amounts in the financial statements have been recast to reflect application of the new guidance. See Note 2 to the Consolidated Financial Statements for additional information.