Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 202.8	$ 123.4
Operating Costs and Expenses [Abstract]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(117.8)	64.4
Impairment Losses	(12.0)	(3.6)
Income from Continuing Operations before Income Taxes	77.7	194.7
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	124.5	132.0
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	75.0	99.4
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	23.0	3.3
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	26.5	29.3
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	78.3	(8.6)
Operating Costs and Expenses [Abstract]
Partial Satisfaction of Judgment	89.4	0.0
Other	(11.1)	(8.6)
Segment Reconciling Items
Operating Costs and Expenses [Abstract]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	(117.8)	64.4
Net Realized Gains on Sales of Investments	16.5	16.1
Impairment Losses	(12.0)	(3.6)
Acquisition Related Transaction, Integration and Other Costs	$ (11.8)	$ (5.6)

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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-Publisher FASB
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This item represents the net total realized and unrealized gain (loss) included in earnings for the period as a result of selling or holding marketable securities categorized as trading, available-for-sale, or held-to-maturity, including the unrealized holding gain (loss) of held-to-maturity securities transferred to the trading security category and the cumulative unrealized gain (loss) which was included in other comprehensive income (a separate component of shareholders' equity) for available-for-sale securities transferred to trading securities during the period. Additionally, this item would include any gains (losses) realized during the period from the sale of investments accounted for under the cost method of accounting.

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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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Amount of income (loss) from continuing operations before deduction of income tax expense (benefit) and income (loss) attributable to noncontrolling interest, and addition of income (loss) from equity method investments.

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The net result for the period of deducting operating expenses from operating revenues.

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The net amount of other operating income and expenses, the components of which are not separately disclosed on the income statement, from items that are associated with the entity's normal revenue producing operations.

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The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

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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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