Investment Income and Gains and Losses (Components of Net Investment Income) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 274.7	$ 290.0	$ 277.6
Less: investment expenses	(9.6)	(8.7)	(10.2)
Net investment income	265.1	281.3	267.4
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	222.5	232.4	217.7
Limited Partnerships [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	16.7	19.7	24.1
Mortgage Loans [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	17.5	16.3	14.0
Equity Securities [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	14.8	16.3	17.0
Other Investments [Member]
Gain (Loss) on Investments [Line Items]
Gross investment income	$ 3.2	$ 5.3	$ 4.8

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
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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)
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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 http://asc.fasb.org/extlink&oid=120400993&loc=SL114874131-224263

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+ References

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