Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,031.9	$ 1,180.9
Net Investment Income	100.4	101.8
Change in Value of Alternative Energy Partnership Investments	0.4	0.7
Other Income	1.8	1.2
Total Revenues	1,143.0	1,294.8
Change in Fair Value of Equity and Convertible Securities	3.4	1.7
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,024.4	1,132.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	840.0	944.2
Net Investment Income	41.1	38.5
Change in Value of Alternative Energy Partnership Investments	0.3	0.4
Other Income	1.1	0.9
Total Revenues	882.5	984.0
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	94.6	137.4
Total Revenues	106.2	148.0
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	97.3	99.3
Net Investment Income	44.3	49.8
Change in Value of Alternative Energy Partnership Investments	0.1	0.2
Other Income	0.2	(0.4)
Total Revenues	141.9	148.9
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	3.4	1.7
Gain (Loss) on Sale of Investments	6.6	6.4
Net Impairment Losses Recognized in Earnings	(1.5)	2.1
Other	$ 3.9	$ 3.7