Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 75.9	$ 202.8
Corporate and Other Operating Income (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	52.2	(117.8)
Impairment Losses	(4.0)	(12.0)
Income before Income Taxes	121.6	77.7
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	87.4	124.5
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	85.2	75.0
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	3.0	23.0
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(0.8)	26.5
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(11.5)	78.3
Corporate and Other Operating Income (Loss) From:
Partial Satisfaction of Judgment	0.0	89.4
Other	(11.5)	(11.1)
Segment Reconciling Items
Corporate and Other Operating Income (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	52.2	(117.8)
Net Realized Gains on Sales of Investments	13.8	16.5
Impairment Losses	(4.0)	(12.0)
Acquisition Related Transaction, Integration and Other Costs	$ (16.3)	$ (11.8)

Business Combination, Acquisition, Integration, and Other Related Costs, Before Tax

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Amount of unrealized and realized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Amount of gain (loss) recognized in settlement of litigation and insurance claims. Excludes claims within an insurance entity's normal claims settlement process.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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