Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Due in One Year or Less, Amortized Cost	$ 130.3
Due in One Year or Less, Fair Value	131.0
Due after One Year to Five Years, Amortized Cost	914.3
Due after One Year to Five Years, Fair Value	917.7
Due after Five Years to Ten Years, Amortized Cost	1,816.8
Due after Five Years to Ten Years, Fair Value	1,842.5
Due after Ten Years, Amortized Cost	2,234.8
Due after Ten Years, Fair Value	2,358.9
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost	1,188.3
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	1,174.1
Amortized Cost	6,284.5	$ 5,021.6
Investments in Fixed Maturities, Fair Value	6,424.2	$ 5,382.7
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Debt Securities, Available-for-sale [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	618.5
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Debt Securities, Available-for-sale [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	8.4
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Debt Securities, Available-for-sale [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	7.6
Other Non-Governmental Issuers [Member]
Debt Securities, Available-for-sale [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	$ 539.6

Amortized cost of 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 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Topic 320
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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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