Investment Income and Gains and Losses (Proceeds from Sale of Available for Sale Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fixed Maturities Excluding Held-For-Sale Chaucer [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	$ 296.5	$ 306.8	$ 348.2
Gross Gains	2.1	8.1	10.0
Gross Losses	8.4	7.8	5.1
Fixed Maturities Held-For-Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	276.8	188.0	215.6
Gross Gains	1.7	4.1	1.7
Gross Losses	1.7	0.4	1.9
Fixed Maturities Including Held For Sale [Member]
Gain (Loss) on Investments [Line Items]
Proceeds from Sales	573.3	494.8	563.8
Gross Gains	3.8	12.2	11.7
Gross Losses	$ 10.1	$ 8.2	$ 7.0

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=82897076&loc=d3e27357-111563

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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=109979856&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=109223946&loc=d3e3151-108585

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-Topic 230
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-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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