Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Segment Reporting Information [Line Items]
Earned Premiums	$ 5,266.3	$ 5,253.7	$ 4,672.2
Net Investment Income	422.6	427.3	348.2
Other Income	9.2	4.8	94.6
Total Revenues	5,576.8	5,793.0	5,205.7
(Loss) Income from Change in Fair Value of Equity and Convertible Securities	(79.9)	114.6	72.1
Net Realized Investment Gains	4.3	64.8	38.1
Net Impairment Losses Recognized in Earnings	(25.8)	(11.0)	(19.5)
Change in Value of Alternative Energy Partnership Investments	(19.9)	(61.2)	0.0
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	5,658.7	5,619.2	5,025.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	4,046.4	3,948.5	3,335.3
Net Investment Income	140.7	152.5	114.1
Other Income	6.0	4.1	1.8
Total Revenues	4,183.2	4,076.1	3,451.2
Change in Value of Alternative Energy Partnership Investments	(9.9)	(29.0)	0.0
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	595.5	651.7	688.2
Net Investment Income	49.7	68.6	37.7
Other Income	0.0	0.0	0.1
Total Revenues	640.5	704.0	726.0
Change in Value of Alternative Energy Partnership Investments	(4.7)	(16.3)	0.0
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	624.4	653.5	648.7
Net Investment Income	216.5	202.7	198.8
Other Income	(0.6)	(1.3)	0.6
Total Revenues	835.0	839.1	848.1
Change in Value of Alternative Energy Partnership Investments	(5.3)	(15.8)	0.0
Segment Reconciling Items
Segment Reporting Information [Line Items]
(Loss) Income from Change in Fair Value of Equity and Convertible Securities	(79.9)	114.6	72.1
Net Realized Investment Gains	4.3	64.8	38.1
Net Impairment Losses Recognized in Earnings	(25.8)	(11.0)	(19.5)
Other	$ 19.5	$ 5.4	$ 89.7