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

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Amount of gain (loss) recognized in net periodic benefit (cost) credit from irrevocable action relieving primary responsibility for benefit obligation and eliminating risk related to obligation and assets used to effect settlement.

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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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Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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

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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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Business Segments - Segment Operating Profit (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									$ 546.8	$ 518.8	$ 286.2
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	$ 73.1	$ 45.2	$ 71.6	$ (117.8)	$ 39.2	$ 9.8	$ 25.5	$ 64.4	72.1	138.9	(64.3)
Impairment Losses	0.5	(1.0)	(7.0)	(12.0)	(1.7)	(1.8)	(6.7)	(3.6)	(19.5)	(13.8)	(4.5)
Settlement Expense									(64.1)	0.0	0.0
Loss from Early Extinguishment of Debt						(5.8)			0.0	(5.8)	0.0
Income from Continuing Operations before Income Taxes	$ 120.6	$ 152.1	$ 159.7	$ 77.7	$ 156.9	$ 161.5	$ 148.5	$ 194.7	510.1	661.6	199.1
Operating Segments
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									493.9	530.1	290.1
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									420.9	355.9	145.6
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									1.8	52.3	28.6
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									71.2	121.9	115.9
Corporate, Non-Segment
Segment Reporting Information [Line Items]
Segment Operating Profit (Loss)									52.9	(11.3)	(3.9)
Partial Satisfaction of Judgment									89.4	20.1	35.7
Other									(36.5)	(31.4)	(39.6)
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities									72.1	138.9	(64.3)
Net Realized Gains on the Sales of Investments									38.1	41.9	26.4
Impairment Losses									(19.5)	(13.8)	(4.5)
Acquisition Related Transaction, Integration and Other Costs									(63.3)	(18.4)	(44.7)
Settlement Expense									$ (64.1)	$ 0.0	$ 0.0