Investments (Investments Classified by Contractual Maturity Date) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Cost	$ 184.8
Due after one year through five years, Amortized Cost	1,797.6
Due after five years through ten years, Amortized Cost	2,464.5
Due after ten years, Amortized Cost	294.0
Gross fixed maturities, Amortized Cost	4,740.9
Mortgage-backed and asset-backed securities, Amortized Cost	1,463.4
Amortized Cost	6,204.3	$ 5,657.1
Fixed maturities, held-for-sale	1,841.7
Total fixed maturities	8,046.0
Due in one year or less, Fair Value	186.5
Due after one year through five years, Fair Value	1,798.9
Due after five years through ten years, Fair Value	2,389.7
Due after ten years, Fair Value	294.6
Gross fixed maturities, Fair Value	4,669.7
Mortgage-backed and asset-backed securities, Fair Value	1,422.9
Fixed maturities, excluding held-for-sale (Chaucer)	6,092.6	$ 5,749.3
Fixed maturities, held-for-sale	1,806.9
Total fixed maturities	$ 7,899.5