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, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.

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Amount of revenue and income classified as other.

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Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

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Business Segments - Segment Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Earned Premiums	$ 1,145.8	$ 1,135.2	$ 1,116.6	$ 1,074.8	$ 1,063.6	$ 1,052.9	$ 658.1	$ 609.8	$ 4,472.4	$ 3,384.4	$ 2,350.0
Net Investment Income	93.9	91.7	96.0	82.7	91.3	92.0	78.4	79.2	364.3	340.9	327.2
Other Income	3.7	7.2	22.7	1.9	2.0	37.8	1.2	1.2	35.5	42.2	4.0
Total Revenues	1,283.7	1,243.8	1,275.4	1,236.3	1,094.7	1,195.5	741.9	693.0	5,039.2	3,725.1	2,723.4
Income (Loss) from Change in Fair Value of Equity and Convertible Securities	39.2	9.8	25.5	64.4	(76.4)	11.0	0.4	0.7	138.9	(64.3)	0.0
Net Impairment Losses Recognized in Earnings	$ (1.7)	$ (1.8)	$ (6.7)	$ (3.6)	$ (2.2)	$ (1.8)	$ 0.0	$ (0.5)	(13.8)	(4.5)	(14.3)
Operating Segments
Segment Reporting Information [Line Items]
Total Revenues									4,845.9	3,726.9	2,675.0
Operating Segments \| Specialty Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums									3,078.4	2,027.4	1,005.7
Net Investment Income									107.5	63.4	39.2
Other Income									7.0	2.4	1.1
Total Revenues									3,192.9	2,093.2	1,046.0
Operating Segments \| Preferred Property & Casualty Insurance
Segment Reporting Information [Line Items]
Earned Premiums									750.3	730.7	730.3
Net Investment Income									44.1	61.8	58.9
Total Revenues									794.4	792.5	789.2
Operating Segments \| Life & Health Insurance
Segment Reporting Information [Line Items]
Earned Premiums									643.7	626.3	614.0
Net Investment Income									206.4	210.9	223.2
Other Income									8.5	4.0	2.6
Total Revenues									858.6	841.2	839.8
Segment Reconciling Items
Segment Reporting Information [Line Items]
Income (Loss) from Change in Fair Value of Equity and Convertible Securities									138.9	(64.3)	0.0
Net Realized Gains on the Sales of Investments									41.9	26.4	56.5
Net Impairment Losses Recognized in Earnings									(13.8)	(4.5)	(14.3)
Other									$ 26.3	$ 40.6	$ 6.2