Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,307.0	$ 1,356.1	$ 3,999.3	$ 3,894.6
Net Investment Income	97.8	101.9	316.3	318.9
Change in Value of Alternative Energy Partnership Investments	0.4	(23.8)	(21.2)	(46.9)
Other Income (Loss)	4.0	12.3	7.3	20.8
Total Revenues	1,377.6	1,455.4	4,200.1	4,315.1
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(11.2)	(0.6)	(79.9)	92.4
Net Impairment Losses Recognized in Earnings	(8.3)	(0.6)	(22.1)	(7.8)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,408.0	1,435.1	4,285.2	4,169.2
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	999.5	1,028.3	3,064.8	2,916.2
Net Investment Income	33.9	37.0	102.8	114.7
Change in Value of Alternative Energy Partnership Investments	0.3	(11.3)	(10.6)	(22.3)
Other Income (Loss)	2.3	1.2	5.0	3.1
Total Revenues	1,036.0	1,055.2	3,162.0	3,011.7
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	149.3	163.7	454.8	489.1
Net Investment Income	11.8	16.1	36.2	51.5
Change in Value of Alternative Energy Partnership Investments	0.0	(6.4)	(5.0)	(12.5)
Total Revenues	161.1	173.4	486.0	528.1
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	158.2	164.1	479.7	489.3
Net Investment Income	52.6	48.4	163.9	151.9
Change in Value of Alternative Energy Partnership Investments	0.1	(6.1)	(5.6)	(12.1)
Other Income (Loss)	0.0	0.1	(0.8)	0.3
Total Revenues	210.9	206.5	637.2	629.4
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(11.2)	(0.6)	(79.9)	92.4
Gain (Loss) on Sale of Investments	(12.1)	10.1	0.4	43.1
Net Impairment Losses Recognized in Earnings	(8.3)	(0.6)	(22.1)	(7.8)
Other	$ 1.2	$ 11.4	$ 16.5	$ 18.2