Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	$ 1,180.9	$ 1,320.0
Net Investment Income	101.8	100.0
Change in Value of Alternative Energy Partnership Investments	0.7	(16.7)
Other Income	1.2	2.4
Total Revenues	1,294.8	1,370.1
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	1.7	(28.2)
Impairment Gains (Losses)	2.1	(8.9)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,280.9	1,401.8
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	944.2	1,021.6
Net Investment Income	38.5	34.9
Change in Value of Alternative Energy Partnership Investments	0.4	(8.4)
Other Income	0.9	1.7
Total Revenues	984.0	1,049.8
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	137.4	155.6
Net Investment Income	10.5	12.5
Change in Value of Alternative Energy Partnership Investments	0.1	(3.9)
Total Revenues	148.0	164.2
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability: 2023 - $19.2; 2022 - $20.3)	99.3	142.8
Net Investment Income	49.8	49.4
Change in Value of Alternative Energy Partnership Investments	0.2	(4.4)
Other Income	(0.4)	0.0
Total Revenues	148.9	187.8
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	1.7	(28.2)
Gain (Loss) on Sale of Investments	6.4	1.5
Impairment Gains (Losses)	2.1	(8.9)
Other	$ 3.7	$ 3.9