Investments - Schedule of Contractual Maturity (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Due in One Year or Less, Amortized Cost	$ 81.3
Due in One Year or Less, Fair Value	82.9
Due after One Year to Five Years, Amortized Cost	724.8
Due after One Year to Five Years, Fair Value	766.0
Due after Five Years to Ten Years, Amortized Cost	1,304.4
Due after Five Years to Ten Years, Fair Value	1,324.4
Due after Ten Years, Amortized Cost	2,051.1
Due after Ten Years, Fair Value	2,183.4
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost	208.9
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	218.3
Investments in Fixed Maturities, Amortized Cost	4,370.5	4,283.8
Investments in Fixed Maturities, Fair Value	4,575.0	4,860.2
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	50.7
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	155.9
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	11.3
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	$ 0.4