Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ 100.5	$ 109.6	$ 303.3	$ 233.0
Corporate and Other Operating Profit (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	71.6	25.5	(46.2)	89.9
Impairment Losses			(19.0)	(10.3)
Income before Income Taxes	159.7	148.5	237.4	343.2
Operating Segments
Segment Reporting Information [Line Items]
Operating Income (Loss)	105.2	101.9	229.7	233.9
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	84.5	79.4	159.5	178.8
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	0.8	6.4	23.8	9.7
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Operating Income (Loss)	19.9	16.1	46.4	45.4
Corporate and Other
Segment Reporting Information [Line Items]
Operating Income (Loss)	(4.7)	7.7	73.6	(0.9)
Corporate and Other Operating Profit (Loss) From:
Partial Satisfaction of Judgment	0.0	20.1	89.4	20.1
Other	(4.7)	(12.4)	(15.8)	(21.0)
Segment Reconciling Items
Corporate and Other Operating Profit (Loss) From:
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	71.6	25.5	(46.2)	89.9
Net Realized Gains on Sales of Investments	11.7	21.3	28.2	37.4
Impairment Losses	(7.0)	(6.7)	(19.0)	(10.3)
Acquisition Related Transaction, Integration and Other Costs	$ (17.1)	$ (1.2)	$ (28.9)	$ (6.8)

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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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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