Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Segment Net Operating Income	$ 130.0	$ 104.5	$ 320.4	$ 198.5
Loss from Early Extinguishment of Debt	(4.6)	0.0	(4.6)	0.0
Income from Continuing Operations	129.0	92.3	406.4	183.4
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income	133.0	60.8	319.7	164.9
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	21.1	12.2	29.1	19.7
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	78.5	21.5	220.8	67.3
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income	33.4	27.1	69.8	77.9
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income	(3.0)	43.7	0.7	33.6
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	7.8	8.7	78.8	9.6
Net Realized Gains on Sales of Investments	1.4	2.8	30.9	7.9
Net Impairment Losses Recognized in Earnings	(1.5)	(1.4)	(9.6)	(1.8)
Acquisition Related Transaction, Integration and Other Costs	$ (4.1)	$ (22.3)	$ (9.5)	$ (30.8)