Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost, net of Allowance for Credit Losses	$ 334.1
Due after one year through five years, Amortized Cost, net of Allowance for Credit Losses	1,774.6
Due after five years through ten years, Amortized Cost, net of Allowance for Credit Losses	2,306.3
Due after ten years, Amortized Cost, net of Allowance for Credit Losses	775.6
Gross fixed maturities, Amortized Cost, net of Allowance for Credit Losses	5,190.6
Mortgage-backed and asset-backed securities, Amortized Cost, net of Allowance for Credit Losses	1,755.0
Amortized Cost, Net of Allowance for Credit Losses	6,945.6
Due in one year or less, Fair Value	339.1
Due after one year through five years, Fair Value	1,913.3
Due after five years through ten years, Fair Value	2,548.2
Due after ten years, Fair Value	809.1
Gross fixed maturities, Fair Value	5,609.7
Mortgage-backed and asset-backed securities, Fair Value	1,844.7
Fixed maturities, Fair Value	$ 7,454.4	$ 6,687.1