Business Segments - Segment Net Income (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					$ 418.3	$ 258.4	$ 92.5
Loss from Early Extinguishment of Debt					(4.6)	0.0	0.0
Income from Continuing Operations	$ 5.0	$ 92.3	$ 37.5	$ 53.6	531.1	188.4	119.9
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					423.7	233.0	102.8
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					283.1	115.8	56.3
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					41.9	25.7	(45.4)
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					98.7	91.5	91.9
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)					(5.4)	25.4	(10.3)
Effects of Tax Law Changes				$ 7.4	0.0	26.4
Partial Satisfaction of Judgment					15.9	28.2	0.0
Other					(21.3)	(29.2)	(17.7)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities					109.7	(50.8)	0.0
Net Realized Gains on Sales of Investments					33.1	20.9	36.7
Net Impairment Losses Recognized in Earnings					(10.9)	(3.6)	(9.3)
Acquisition Related Transaction, Integration and Other Costs					$ (14.5)	$ (36.5)	$ 0.0