Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Premiums Earned, Net	$ 1,068.5	$ 1,117.8	$ 3,134.1	$ 3,465.6
Net Investment Income	111.1	107.0	304.5	315.1
Change in Value of Alternative Energy Partnership Investments	0.5	0.8	1.5	2.3
Other Income	2.2	2.4	6.4	5.3
Total Revenues	1,178.9	1,199.4	3,451.8	3,757.0
Change in Fair Value of Equity and Convertible Securities	(2.3)	2.8	(0.1)	6.9
Impairment Losses	2.2	1.1	3.8	(0.1)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,123.7	1,086.8	3,190.4	3,347.2
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	919.0	890.4	2,621.6	2,766.9
Net Investment Income	52.0	42.7	139.7	125.7
Change in Value of Alternative Energy Partnership Investments	0.2	0.5	0.8	1.3
Other Income	1.4	1.6	3.8	3.2
Total Revenues	972.6	935.2	2,765.9	2,897.1
Operating Segments | Life Insurance
Segment Reporting Information [Line Items]
Premiums Earned, Net	100.6	102.1	298.7	303.6
Net Investment Income	50.3	49.4	125.1	146.3
Change in Value of Alternative Energy Partnership Investments	0.2	0.2	0.4	0.6
Other Income	0.0	(0.1)	0.3	(0.4)
Total Revenues	151.1	151.6	424.5	450.1
Operating Segments | Non-Core Operations
Segment Reporting Information [Line Items]
Premiums Earned, Net	48.9	125.3	213.8	395.1
Total Revenues	54.8	138.0	244.8	431.7
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	(2.3)	2.8	(0.1)	6.9
Gain (Loss) on Sale of Investments	1.1	(30.3)	9.2	(38.3)
Impairment Losses	(2.2)	(1.1)	3.8	(0.1)
Other Income	$ 3.8	$ 3.2	$ 11.3	$ 9.4