Investments - Schedule of Contractual Maturity (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amortized Cost
Due in One Year or Less	$ 82.5
Due after One Year to Five Years	882.3
Due after Five Years to Ten Years	1,581.7
Due after Ten Years	2,535.3
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,290.9
Amortized Cost	6,372.7	$ 6,284.5
Fair Value
Due in One Year or Less	85.1
Due after One Year to Five Years	909.2
Due after Five Years to Ten Years	1,708.9
Due after Ten Years	2,916.6
Mortgage- and Asset-backed Securities Not Due at a Single Maturity Date	1,302.3
Investments in Fixed Maturities	$ 6,922.1	$ 6,424.2