Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Earned Premiums	$ 5,253.7	$ 4,672.2	$ 4,472.4
Net Investment Income	427.3	348.2	364.3
Other Income (Loss)	4.8	94.6	35.5
Total Revenues	5,793.0	5,205.7	5,039.2
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	114.6	72.1	138.9
Net Realized Gains on the Sales of Investments	64.8	38.1	41.9
Net Impairment Losses Recognized in Earnings	(11.0)	(19.5)	(13.8)
Change in Value of Alternative Energy Partnership Investments	(61.2)	0.0	0.0
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	5,619.2	5,025.3	4,845.9
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	3,948.5	3,335.3	3,078.4
Net Investment Income	152.5	114.1	107.5
Other Income (Loss)	4.1	1.8	7.0
Total Revenues	4,076.1	3,451.2	3,192.9
Change in Value of Alternative Energy Partnership Investments	(29.0)	0.0	0.0
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	651.7	688.2	750.3
Net Investment Income	68.6	37.7	44.1
Other Income (Loss)	0.0	0.1
Total Revenues	704.0	726.0	794.4
Change in Value of Alternative Energy Partnership Investments	(16.3)
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	653.5	648.7	643.7
Net Investment Income	202.7	198.8	206.4
Other Income (Loss)	(1.3)	0.6	8.5
Total Revenues	839.1	848.1	858.6
Change in Value of Alternative Energy Partnership Investments	(15.8)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	114.6	72.1	138.9
Net Realized Gains on the Sales of Investments	64.8	38.1	41.9
Net Impairment Losses Recognized in Earnings	(11.0)	(19.5)	(13.8)
Other	$ 5.4	$ 89.7	$ 26.3