Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Due in One Year or Less, Amortized Cost	$ 152.1
Due in One Year or Less, Fair Value	153.4
Due after One Year to Five Years, Amortized Cost	855.8
Due after One Year to Five Years, Fair Value	879.0
Due after Five Years to Ten Years, Amortized Cost	1,603.3
Due after Five Years to Ten Years, Fair Value	1,678.0
Due after Ten Years, Amortized Cost	1,911.4
Due after Ten Years, Fair Value	2,164.0
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost	499.0
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	508.3
Amortized Cost	5,021.6	$ 4,846.8
Investments in Fixed Maturities, Fair Value	5,382.7	$ 5,124.9
Government National Mortgage Association Certificates and Obligations (GNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	352.9
Federal National Mortgage Association Certificates and Obligations (FNMA) [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	10.7
Federal Home Loan Mortgage Corporation Certificates and Obligations (FHLMC) [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	3.9
Other Non-Governmental Issuers [Member]
Schedule of Available-for-sale Securities [Line Items]
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	$ 140.8