Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (130.6)	$ 112.3	$ (190.8)	$ 415.6
Corporate and Other Operating Income (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(0.6)	45.2	92.4	(1.0)
Debt and Equity Securities, Realized Gain (Loss)	10.1	10.0	43.1	38.2
Impairment Losses	0.6	1.0	7.8	20.0
Income (Loss) before Income Taxes	(129.8)	152.1	(97.9)	389.5
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	(115.1)	123.3	(150.0)	353.0
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(91.8)	149.9	(128.7)	309.4
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(17.4)	(41.8)	(27.8)	(18.0)
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(5.9)	15.2	6.5	61.6
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(15.5)	(11.0)	(40.8)	62.6
Corporate and Other Operating Income (Loss) From:
Partial Satisfaction of Judgment	0.0	0.0	0.0	89.4
Other	(15.5)	(11.0)	(40.8)	(26.8)
Segment Reconciling Items
Corporate and Other Operating Income (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(0.6)	45.2	92.4	(1.0)
Debt and Equity Securities, Realized Gain (Loss)	10.1	10.0	43.1	38.2
Impairment Losses	0.6	1.0	7.8	20.0
Acquisition Related Transaction, Integration and Other Costs	$ (8.1)	$ (14.4)	$ (34.8)	$ (43.3)

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