Investments - Schedule of Contractual Maturity (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Due in One Year or Less, Amortized Cost	$ 64.0
Due in One Year or Less, Fair Value	65.8
Due after One Year to Five Years, Amortized Cost	606.0
Due after One Year to Five Years, Fair Value	639.0
Due after Five Years to Ten Years, Amortized Cost	1,139.2
Due after Five Years to Ten Years, Fair Value	1,251.8
Due after Ten Years, Amortized Cost	2,280.9
Due after Ten Years, Fair Value	2,690.2
Asset-backed Securities Not Due at a Single Maturity Date, Amortized Cost	193.7
Asset-backed Securities Not Due at a Single Maturity Date, Fair Value	213.4
Investments in Fixed Maturities, Amortized Cost	4,283.8
Investments in Fixed Maturities, Fair Value	$ 4,860.2	$ 4,773.4