Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,206.5	$ 1,135.2	$ 3,458.2	$ 3,326.6
Net Investment Income	92.1	91.7	245.5	270.4
Other Income	0.9	7.2	92.7	31.8
Total Revenues	1,353.7	1,243.8	3,813.6	3,755.5
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	45.2	9.8	(1.0)	99.7
Impairment Losses			(20.0)	(12.1)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,298.4	1,233.0	3,706.8	3,604.6
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	871.4	783.4	2,452.9	2,278.7
Net Investment Income	30.5	28.8	76.2	79.2
Other Income	0.4	4.4	1.4	6.2
Total Revenues	902.3	816.6	2,530.5	2,364.1
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	174.5	191.0	519.0	565.1
Net Investment Income	10.3	12.0	24.3	32.6
Other Income	0.0	0.0	0.1	0.0
Total Revenues	184.8	203.0	543.4	597.7
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	160.6	160.8	486.3	482.8
Net Investment Income	50.7	49.7	146.0	154.4
Other Income	0.0	2.9	0.6	5.6
Total Revenues	211.3	213.4	632.9	642.8
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	45.2	9.8	(1.0)	99.7
Net Realized Gains on Sales of Investments	10.0	1.7	38.2	39.1
Impairment Losses	(1.0)	(1.8)	(20.0)	(12.1)
Other	$ 1.1	$ 1.1	$ 89.6	$ 24.2