Investments - Mortgage Loans (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 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	$ 4,110.4	$ 3,730.3
Collective valuation allowance for losses	(1.1)	(1.2)
Total net commercial mortgage loans	4,109.3	3,729.1
Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4.6	10.7
Total net commercial mortgage loans	4.6	10.7
Non Impaired
Accounts, Notes, Loans and Financing Receivable [Line Items]
Total commercial mortgage loans	4,105.8	3,719.6
Collective valuation allowance for losses	(1.1)	(1.2)
Total net commercial mortgage loans	$ 4,104.7	$ 3,718.4