Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Due in One Year or Less, Amortized Cost	$ 101.6
Due in One Year or Less, Fair Value	102.8
Due after One Year to Five Years, Amortized Cost	836.7
Due after One Year to Five Years, Fair Value	867.0
Due after Five Years to Ten Years, Amortized Cost	1,608.6
Due after Five Years to Ten Years, Fair Value	1,652.4
Due after Ten Years, Amortized Cost	2,038.2
Due after Ten Years, Fair Value	2,233.0
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost	261.7
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	269.7
Amortized Cost	4,846.8	$ 4,560.7
Investments in Fixed Maturities, Fair Value	5,124.9	$ 4,852.3
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	126.2
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	13.9
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	4.8
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	$ 124.8