Business Segments - Segment Net Income (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						$ 438.8	$ 418.3	$ 258.4
Loss from Early Extinguishment of Debt						(50.6)	(4.6)	0.0
Income from Continuing Operations	$ 97.5	$ 122.3	$ 126.1	$ 64.0	$ 188.4	409.9	531.1	188.4
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						401.4	423.7	233.0
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						337.9	283.1	115.8
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						3.5	41.9	25.7
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						60.0	98.7	91.5
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)						37.4	(5.4)	25.4
Effects of Tax Law Changes						0.0	0.0	26.4
Partial Satisfaction of Judgment						70.6	15.9	28.2
Other						(33.2)	(21.3)	(29.2)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities						57.0	109.7	(50.8)
Net Realized Gains on Sales of Investments						30.1	33.1	20.9
Impairment Losses						(15.4)	(10.9)	(3.6)
Acquisition Related Transaction, Integration and Other Costs						$ (50.0)	$ (14.5)	$ (36.5)