Separate Accounts, Death Benefits and Other Insurance Benefit Features and Embedded Product Derivatives - Corporate Account Investments of Account Balances of Variable Annuity Contracts with Guarantees (Details) - Variable Annuity - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of separate accounts by major category of investment, fair value	$ 1,485.7	$ 1,614.6
Debt securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of separate accounts by major category of investment, fair value	253.2	276.0
Equity securities
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of separate accounts by major category of investment, fair value	1,199.5	1,303.1
Other
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Fair value of separate accounts by major category of investment, fair value	$ 33.0	$ 35.5