Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost	$ 408.2
Due after one year through five years, Amortized Cost	2,789.5
Due after five years through ten years, Amortized Cost	2,589.6
Due after ten years, Amortized Cost	331.4
Gross fixed maturities, Amortized Cost	6,118.7
Mortgage-backed and asset-backed securities, Amortized Cost	1,729.1
Amortized Cost or Cost	7,847.8	$ 7,688.8
Due in one year or less, Fair Value	409.7
Due after one year through five years, Fair Value	2,784.6
Due after five years through ten years, Fair Value	2,515.2
Due after ten years, Fair Value	334.5
Gross fixed maturities, Fair Value	6,044.0
Mortgage-backed and asset-backed securities, Fair Value	1,685.9
Fixed maturities, Fair Value	$ 7,729.9	$ 7,779.7