Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	$ 1,117.8	$ 1,290.9	$ 3,465.6	$ 3,948.5
Net Investment Income	107.0	97.8	315.1	316.3
Change in Value of Alternative Energy Partnership Investments	0.8	0.4	2.3	(21.2)
Other Income	2.4	4.0	5.3	7.3
Total Revenues	1,199.4	1,361.5	3,757.0	4,149.3
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	2.8	(11.2)	6.9	(79.9)
Impairment Gains (Losses)	(1.1)	(8.3)	0.1	(22.1)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,086.8	1,230.8	3,347.2	3,748.4
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	890.4	999.5	2,766.9	3,064.8
Net Investment Income	42.7	33.9	125.7	102.8
Change in Value of Alternative Energy Partnership Investments	0.5	0.3	1.3	(10.6)
Other Income	1.6	2.3	3.2	5.0
Total Revenues	935.2	1,036.0	2,897.1	3,162.0
Operating Segments | Preferred Property & Casualty Insurance [Member]
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	125.3	149.3	395.1	454.8
Total Revenues	138.0	161.1	431.7	486.0
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Earned Premiums (Changes in Deferred Profit Liability for the Nine and Three Months Ended: 2023 - $51.6 and $15.9; 2022 - $56.8 and $19.1)	102.1	142.1	303.6	428.9
Net Investment Income	49.4	52.6	146.3	163.9
Change in Value of Alternative Energy Partnership Investments	0.2	0.1	0.6	(5.6)
Other Income	(0.1)	0.0	(0.4)	(0.8)
Total Revenues	151.6	194.8	450.1	586.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	2.8	(11.2)	6.9	(79.9)
Gain (Loss) on Sale of Investments	(30.3)	(12.1)	(38.3)	0.4
Impairment Gains (Losses)			0.1	(22.1)
Net Impairment Gains (Losses) Recognized in Earnings	(1.1)	(8.3)
Other	$ 3.2	$ 1.2	$ 9.4	$ 16.5