Investment Income and Gains and Losses (Components of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 307.7	$ 321.7	$ 274.7
Less: investment expenses	(11.4)	(11.0)	(9.6)
Net investment income	296.3	310.7	265.1
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	239.3	216.9	222.5
Limited Partnerships Interest [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	35.7	68.2	16.7
Mortgage Loans on Real Estate [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	16.1	18.0	17.5
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	12.0	15.6	14.8
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 4.6	$ 3.0	$ 3.2

Amount before accretion (amortization) of purchase discount (premium) of interest income and dividend income on nonoperating securities.

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Amount of expenses related to the generation of investment income.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section S99
-Paragraph 2
-Subparagraph (SX 210.5-03.9)
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.2)
-URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04(2))
-URI https://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

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+ References

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