Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 112.3	$ 163.0	$ 415.6	$ 396.0
Corporate and Other Operating Profit (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	45.2	9.8	(1.0)	99.7
Impairment Losses			(20.0)	(12.1)
Loss from Early Extinguishment of Debt	0.0	(5.8)	0.0	(5.8)
Income before Income Taxes	152.1	161.5	389.5	504.7
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	123.3	166.1	353.0	400.0
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	149.9	98.6	309.4	277.4
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	(41.8)	26.5	(18.0)	36.2
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	15.2	41.0	61.6	86.4
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(11.0)	(3.1)	62.6	(4.0)
Corporate and Other Operating Profit (Loss) From:
Partial Satisfaction of Judgment	0.0	0.0	89.4	20.1
Other	(11.0)	(3.1)	(26.8)	(24.1)
Segment Reconciling Items
Corporate and Other Operating Profit (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	45.2	9.8	(1.0)	99.7
Net Realized Gains on Sales of Investments	10.0	1.7	38.2	39.1
Impairment Losses	(1.0)	(1.8)	(20.0)	(12.1)
Acquisition Related Transaction, Integration and Other Costs	(14.4)	(5.4)	(43.3)	(12.2)
Loss from Early Extinguishment of Debt	$ 0.0	$ (5.8)	$ 0.0	$ (5.8)

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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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, before tax, of income (loss) attributable to parent. Includes, but is not limited to, income (loss) from continuing operations, discontinued operations and 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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