Investment Securities (Cost and Amortized Cost Maturity Schedule) (Details) $ in Millions	Dec. 31, 2015 USD ($)
Trading Securities by Maturity Cost or Amortized Cost
Cost or amortized cost	$ 2,639.1
Trading Securities by Maturity, Fair Value
Trading Securities	2,639.7
Fixed Maturities and Convertible Fixed Maturities Excluding A B S M B S and Preferred Stock
Trading Securities by Maturity Cost or Amortized Cost
Due in one year or less	241.6
Due after one year through five years	982.3
Due after five years through ten years	96.4
Due after ten years	69.9
Trading Securities by Maturity, Fair Value
Due in one year or less	242.0
Due after one year through five years	981.5
Due after five years through ten years	96.1
Due after ten years	70.4
Mortgage-backed and asset-backed securities
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	1,170.6
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	1,167.0
Preferred stocks
Trading Securities by Maturity Cost or Amortized Cost
Trading Securities, Maturities without Single Maturity Date, Amortized Cost	78.3
Trading Securities by Maturity, Fair Value
Trading Securities, Maturities without Single Maturity Date, Carrying Value	$ 82.7