Investment Income and Gains and Losses (Proceeds from Sales of Available for Sale Fixed Maturities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 264.1
Gross Gains	9.9
Gross Losses	$ 2.9
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		$ 541.2	$ 296.5
Gross Gains		6.9	2.1
Gross Losses		5.7	8.4
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		0.3	276.8
Gross Gains			1.7
Gross Losses			1.7
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales		541.5	573.3
Gross Gains		6.9	3.8
Gross Losses		$ 5.7	$ 10.1

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27357-111563

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121553693&loc=d3e26853-111562

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 11
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3151-108585

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-Paragraph 12
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