Investments - Mortgage Loans (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Investments, Debt and Equity Securities [Abstract]
Maximum loan to value ratio generally allowed	75.00%
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	$ 3,931.2	$ 3,730.3
Collective valuation allowance for losses	(1.2)	(1.2)
Total net commercial mortgage loans	3,930.0	3,729.1
Impairments on mortgage loans	0.0
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4.8	10.7
Total net commercial mortgage loans	4.8	10.7
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	3,926.4	3,719.6
Collective valuation allowance for losses	(1.2)	(1.2)
Total net commercial mortgage loans	$ 3,925.2	$ 3,718.4