Business Segments - Segment Net Operating Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	$ (99.4)	$ 79.2	$ (12.2)	$ 242.1
Net Income	(62.6)	126.1	60.6	190.1
Operating Segments
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(87.0)	84.5	10.0	185.3
Operating Segments | Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(91.7)	67.5	(11.6)	127.6
Operating Segments | Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(8.3)	0.9	1.3	19.3
Operating Segments | Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	13.0	16.1	20.3	38.4
Corporate and Other
Segment Reporting Information [Line Items]
Segment Net Operating Income (Loss)	(12.4)	(5.3)	(22.2)	56.8
Partial Satisfaction of Judgment	0.0	0.0	0.0	70.6
Other	(12.4)	(5.3)	(22.2)	(13.8)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Change in Fair Value of Equity and Convertible Securities	32.3	56.6	73.5	(36.5)
Net Realized Gains on Sales of Investments	15.2	9.3	26.1	22.3
Impairment Losses	(2.5)	(5.5)	(5.7)	(15.0)
Acquisition Related Transaction, Integration and Other Costs	$ (8.2)	$ (13.5)	$ (21.1)	$ (22.8)