Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,166.4	$ 1,074.8
Net Investment Income	85.6	82.7
Other Income	90.3	1.9
Total Revenues	1,229.0	1,236.3
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(117.8)	64.4
Impairment Losses	(12.0)	(3.6)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues	1,256.9	1,158.2
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	822.5	729.3
Net Investment Income	28.8	21.5
Other Income	0.9	0.8
Total Revenues	852.2	751.6
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums	180.9	185.6
Net Investment Income	9.7	8.3
Total Revenues	190.6	193.9
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums	163.0	159.9
Net Investment Income	51.0	51.7
Other Income	0.1	1.1
Total Revenues	214.1	212.7
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(117.8)	64.4
Net Realized Gains on Sales of Investments	16.5	16.1
Impairment Losses	(12.0)	(3.6)
Other	$ 85.4	$ 1.2